feat: add 'photobook' theme with multi-view modes and full-screen viewer

Introduces a new theme "photobook" optimized for high-fidelity media
presentation and print-ready PDF export.

* **Three Integrated View Modes:**
    * **Outline:** A thumbnail grid for rapid navigation.
    * **Slides:** A paginated, one-per-page presentation.
    * **Photo Book:** A continuous, full-width scrolling view optimized
      for printing.
* **Full-Screen Media Viewer:**
    * Triggered by clicking media in Slides mode with a "Click to view
      full size" hint.
    * Includes a dark overlay (95% opacity) and smooth fade-in effects.
    * Full keyboard support (Arrows, Space, ESC, Home, End).
* **Enhanced Metadata Display (Rich Captions):**
    * Displays Title, Markdown-sourced descriptions, and Filename.
    * Comprehensive EXIF data: Camera make/model, ISO, exposure,
      aperture, focal length, GPS coordinates and timestamp.
    * Configurable GPS links: Supports both OpenStreetMap and Google
      Maps.

* **Responsive Design:** Implements a two-column desktop layout that
  stacks for tablet and mobile devices.
* **Typography & Styling:** Standardized on Helvetica Neue with a clean
  white (#ffffff) background and UTF-8 charset declaration.
* **Print Optimization:** Added specific page-break logic for the Photo
  Book view to facilitate PDF/Physical book creation.
* **User Interface:** Added navigation buttons, image counters (e.g., "3
  / 25"), and independent scrolling for long description texts.

To enable the new functionality, users should update their configuration
to use
the `photobook` theme. The system automatically handles EXIF extraction
from
JPG, JPEG, and HEIC files.

Author: howardrotterdam

src/sigal/themes/photobook/README.md

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+# Photobook Theme for Sigal
+
+A beautiful, print-friendly photo gallery theme presenting your images in a modern photo book layout.
+
+## Features
+
+- **Photo Book View**: Browse photos in a paginated, magazine-style layout
+- **Outline View**: See all photos as thumbnails in a grid for quick navigation
+- **Rich Captions**: Display title, description, filename, and EXIF metadata
+- **GPS Support**: Clickable GPS coordinates link to OpenStreetMap
+- **Keyboard Navigation**: 
+  - Arrow Right / Space: Next page
+  - Arrow Left: Previous page
+  - Home: First page
+  - End: Last page
+- **Responsive Design**: Works beautifully on desktop, tablet, and mobile
+- **Print-Friendly**: Optimized for printing physical photo books
+- **UTF-8 Encoding**: Full Unicode support for all text
+- **Helvetica Typography**: Clean, professional Helvetica Neue font
+
+## Installation
+
+1. The photobook theme is installed as part of Sigal
+2. Reference it in your `sigal.conf.py`:
+
+```python
+theme = 'photobook'
+```
+
+## Configuration
+
+Add these settings to your `sigal.conf.py` to customize the photobook theme:
+
+```python
+# Theme name
+theme = 'photobook'
+
+# Photos per page (default: 1)
+# Note: Currently displays 1 photo per page
+photobook_photos_per_page = 1
+
+# Enable keyboard navigation (default: True)
+photobook_keyboard_nav = True
+
+# Enable outline click navigation (default: True)
+photobook_outline_nav = True
+
+# Show album description (default: True)
+photobook_show_description = True
+
+# Show EXIF data (default: True)
+photobook_show_exif = True
+
+# Show GPS coordinates (default: True)
+photobook_show_gps = True
+
+# Map provider for GPS coordinates (default: 'openstreetmap')
+# Options: 'openstreetmap' or 'googlemaps'
+photobook_map_provider = 'openstreetmap'
+```
+
+## Configuration Details
+
+### Map Provider
+
+The theme supports two map providers for GPS coordinates:
+
+- **OpenStreetMap** (default): Free, open-source mapping service
+  - Set: `photobook_map_provider = 'openstreetmap'`
+  - Links open to: `https://www.openstreetmap.org/`
+
+- **Google Maps**: Google's mapping service (requires internet access)
+  - Set: `photobook_map_provider = 'googlemaps'`
+  - Links open to: `https://www.google.com/maps/`
+
+You can also change it at runtime using JavaScript:
+
+```javascript
+// Switch to Google Maps
+Photobook.setMapProvider('googlemaps');
+
+// Switch back to OpenStreetMap
+Photobook.setMapProvider('openstreetmap');
+
+// Get current map provider
+var current = Photobook.getMapProvider();
+```
+
+## Usage
+
+### Document Structure
+
+The photobook theme expects media metadata to be provided by Sigal:
+
+- **Title**: Image filename or custom title
+- **Description**: From markdown files or YAML frontmatter
+- **EXIF Data**: Automatically extracted from image files (JPG, JPEG, HEIC)
+  - Camera make and model
+  - ISO, exposure time, aperture, focal length
+  - Date/time taken
+  - GPS location (if available)
+- **Filename**: Full filename with extension
+
+### Adding Descriptions
+
+Create a markdown file with the same name as your image for descriptions:
+
+```
+photo.jpg
+photo.md
+```
+
+Content of `photo.md`:
+```markdown
+# Photo Title (optional)
+
+This is a detailed description of the photo that appears below the image.
+Can include multiple paragraphs and formatting.
+```
+
+### Organizing Photos
+
+Place photos in albums (directories):
+
+```
+gallery/
+├── album1/
+│   ├── photo1.jpg
+│   ├── photo1.md
+│   ├── photo2.jpg
+│   └── photo2.md
+└── album2/
+    ├── trip1.jpg
+    └── trip1.md
+```
+
+## Features Explained
+
+### Two View Modes
+
+1. **Outline View**: Grid of all thumbnails with titles. Click any thumbnail to jump to that photo in book view.
+2. **Photo Book View**: Full-screen image with detailed caption including:
+   - Photo title
+   - Description (from markdown)
+   - Filename
+   - Camera information
+   - Settings (ISO, exposure, aperture, focal length)
+   - GPS location (clickable link to OpenStreetMap)
+
+### Responsive Behavior
+
+- **Desktop**: Two-column layout with image on left, caption on right
+- **Tablet**: Single column, image above caption
+- **Mobile**: Optimized single column layout with readable font sizes
+
+### Print Support
+
+The theme includes print stylesheets for generating PDF photo books:
+
+```bash
+# In your browser, use Print to PDF:
+# - File > Print (Ctrl+P / Cmd+P)
+# - Select "Save as PDF"
+# - Recommended paper size: A4 or Letter
+# - Margins: 0.5 inch
+```
+
+## Styling and Customization
+
+### Colors
+
+The theme uses a clean white background with these main colors:
+
+- Background: `#ffffff` (white)
+- Text: `#333333` (dark gray)
+- Borders: `#dddddd` (light gray)
+- Links: `#0066cc` (blue)
+- Accents: `#f5f5f5` (light gray)
+
+### Font Sizes
+
+- Header: 32px
+- Album title: 28px
+- Photo title: 22px
+- Body text: 16px (15px on tablets, 14px on mobile)
+- Metadata: 14px
+- Small text: 13px
+
+### Custom CSS
+
+Override theme styles with a custom CSS file:
+
+```python
+# In sigal.conf.py
+theme = 'photobook'
+photobook_custom_css = 'my_custom.css'
+```
+
+Create `my_custom.css` in your gallery directory with custom rules:
+
+```css
+.photobook-caption h3 {
+    color: #cc0000;  /* Make titles red */
+}
+
+.photobook-image {
+    border: 3px solid #333;  /* Add border to images */
+}
+
+body {
+    font-size: 18px;  /* Increase base font size */
+}
+```
+
+## Browser Support
+
+- Chrome/Chromium 60+
+- Firefox 55+
+- Safari 11+
+- Edge 79+
+- Mobile browsers (iOS Safari, Chrome Mobile)
+
+## Keyboard Shortcuts
+
+| Key | Action |
+|-----|--------|
+| Right Arrow | Next page |
+| Space | Next page |
+| Left Arrow | Previous page |
+| Home | First page |
+| End | Last page |
+
+## JavaScript API
+
+For advanced customization, the theme exposes a JavaScript API:
+
+```javascript
+// Go to specific page (1-indexed)
+Photobook.goToPage(5);
+
+// Navigate
+Photobook.nextPage();
+Photobook.previousPage();
+
+// Get current state
+var current = Photobook.getCurrentPage();  // Returns 1-indexed page
+var total = Photobook.getTotalPages();
+
+// Configure pages
+Photobook.setPhotosPerPage(2);
+
+// Map provider control
+Photobook.setMapProvider('googlemaps');     // Switch to Google Maps
+Photobook.setMapProvider('openstreetmap');  // Switch to OpenStreetMap
+var provider = Photobook.getMapProvider();  // Get current provider
+```
+
+## Performance Notes
+
+- Lazy loading of images is handled by browsers
+- Works well with 100-500 photos per album
+- Large albums (500+) may benefit from splitting into multiple smaller albums
+
+## Accessibility
+
+- Semantic HTML structure
+- Alt text for all images
+- Keyboard navigation support
+- High contrast text for readability
+- Descriptive link text (GPS coordinates link to location names)
+
+## License
+
+This theme is part of Sigal and follows the same license terms.
+
+## Support
+
+For issues or feature requests, visit:
+https://github.com/saimon-res/sigal

src/sigal/themes/photobook/static/css/colorbox.css

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+/*
+    Colorbox Core Style:
+    The following CSS is consistent between example themes and should not be altered.
+*/
+#colorbox, #cboxOverlay, #cboxWrapper{position:absolute; top:0; left:0; z-index:9999; overflow:hidden; -webkit-transform: translate3d(0,0,0);}
+#cboxWrapper {max-width:none;}
+#cboxOverlay{position:fixed; width:100%; height:100%;}
+#cboxMiddleLeft, #cboxBottomLeft{clear:left;}
+#cboxContent{position:relative;}
+#cboxLoadedContent{overflow:auto; -webkit-overflow-scrolling: touch;}
+#cboxTitle{margin:0;}
+#cboxLoadingOverlay, #cboxLoadingGraphic{position:absolute; top:0; left:0; width:100%; height:100%;}
+#cboxPrevious, #cboxNext, #cboxClose, #cboxSlideshow{cursor:pointer;}
+.cboxPhoto{float:left; margin:auto; border:0; display:block; max-width:none; -ms-interpolation-mode:bicubic;}
+.cboxIframe{width:100%; height:100%; display:block; border:0; padding:0; margin:0;}
+#colorbox, #cboxContent, #cboxLoadedContent{box-sizing:content-box; -moz-box-sizing:content-box; -webkit-box-sizing:content-box;}
+
+/*
+    User Style:
+    Change the following styles to modify the appearance of Colorbox.  They are
+    ordered & tabbed in a way that represents the nesting of the generated HTML.
+*/
+#cboxOverlay{background:#000; opacity: 0.9; filter: alpha(opacity = 90);}
+#colorbox{outline:0;}
+    #cboxContent{margin-top:20px;background:#000;}
+        .cboxIframe{background:#fff;}
+        #cboxError{padding:50px; border:1px solid #ccc;}
+        #cboxLoadedContent{border:5px solid #000; background:#fff;}
+        #cboxTitle{position:absolute; top:-20px; left:0; color:#ccc;}
+        #cboxCurrent{position:absolute; top:-20px; right:0px; color:#ccc;}
+        #cboxLoadingGraphic{background:url(../images/loading.gif) no-repeat center center;}
+
+        /* these elements are buttons, and may need to have additional styles reset to avoid unwanted base styles */
+        #cboxPrevious, #cboxNext, #cboxSlideshow, #cboxClose {border:0; padding:0; margin:0; overflow:visible; width:auto; background:none; }
+
+        /* avoid outlines on :active (mouseclick), but preserve outlines on :focus (tabbed navigating) */
+        #cboxPrevious:active, #cboxNext:active, #cboxSlideshow:active, #cboxClose:active {outline:0;}
+
+        #cboxSlideshow{position:absolute; top:-20px; right:90px; color:#fff;}
+        #cboxPrevious{position:absolute; top:50%; left:5px; margin-top:-32px; background:url(../images/controls.png) no-repeat top left; width:28px; height:65px; text-indent:-9999px;}
+        #cboxPrevious:hover{background-position:bottom left;}
+        #cboxNext{position:absolute; top:50%; right:5px; margin-top:-32px; background:url(../images/controls.png) no-repeat top right; width:28px; height:65px; text-indent:-9999px;}
+        #cboxNext:hover{background-position:bottom right;}
+        #cboxClose{position:absolute; top:5px; right:5px; display:block; background:url(../images/controls.png) no-repeat top center; width:38px; height:19px; text-indent:-9999px;}
+        #cboxClose:hover{background-position:bottom center;}