Add Zvuk Music provider documentation (#79)

## Summary

- Add documentation for the new Zvuk Music provider
- Add Zvuk Music icon to assets

## Changes

- `docs/music-providers/zvuk.md` - Provider documentation including:
  - Feature table with supported capabilities
  - Token-based authentication instructions
  - Audio quality settings
  - Known issues and limitations
- `docs/assets/icons/zvuk-icon.svg` - Provider icon

## Related

This documentation accompanies the Zvuk Music provider implementation.

---

🤖 Generated with [Claude Code](https://claude.ai/claude-code)

Author: OzGav

docs/assets/icons/zvuk-icon.svg

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+# Zvuk Music ![Preview image](../assets/icons/zvuk-icon.svg){ width=50 align=right }
+
+Music Assistant has support for [Zvuk Music](https://zvuk.com). Contributed and maintained by [TrudenBoy](https://github.com/TrudenBoy).
+
+This provider is built on top of the [zvuk-music](https://github.com/trudenboy/zvuk-music) library.
+
+!!! note
+    A Zvuk Music subscription is required for lossless (FLAC) quality. Free accounts can stream at high quality (320 kbps) with limitations.
+
+## Features
+
+|           |                     |
+|:-----------------------|:---------------------:|
+| Subscription FREE | Yes (with limitations) |
+| Self-Hosted Local Media | No |
+| Media Types Supported | Artists, Albums, Tracks, Playlists |
+| [Recommendations](../ui.md#view-home) Supported | No |
+| Lyrics Supported | No |
+| [Radio Mode](../ui.md#track-menu) | No |
+| Maximum Stream Quality | Lossless FLAC (with subscription) |
+| Login Method | Token |
+
+### Other
+
+- Searching the Zvuk Music catalogue is possible
+- Items in a users Zvuk Music library will be synced to Music Assistant
+- Adding/removing items to/from the Music Assistant library will sync back to Zvuk Music
+- Playlist creation and editing is supported
+
+## Configuration
+
+Configuration requires obtaining an X-Auth-Token from Zvuk Music.
+
+### Obtaining the Token
+
+The Zvuk Music provider requires an authentication token (X-Auth-Token) from your Zvuk account.
+
+**Steps:**
+
+1. **Log in** to your Zvuk Music account at [zvuk.com](https://zvuk.com) using your web browser
+
+2. **Navigate** to the profile API endpoint: [https://zvuk.com/api/tiny/profile](https://zvuk.com/api/tiny/profile)
+
+   Your browser will display a JSON response containing your profile information and authentication token.
+
+3. **Locate the token** in the JSON response
+
+   The response will look similar to this:
+
+   ```json
+   {
+     "user": {
+       "id": 12345678,
+       "email": "your@email.com",
+       "token": "abc123def456ghi789jkl012mno345pqr678stu901vwx234yz",
+       ...
+     }
+   }
+   ```
+
+4. **Copy the token value**
+
+   - Find the line with `"token":` in the JSON
+   - Copy only the alphanumeric string between the quotes (not including the quotes)
+   - The token is typically a long string of random letters and numbers
+   - Example token: `abc123def456ghi789jkl012mno345pqr678stu901vwx234yz`
+
+5. **Paste the token** into Music Assistant
+
+   - Go to Music Assistant Settings → Music Providers → Zvuk Music
+   - Paste the token into the "X-Auth-Token" field
+   - Save the configuration
+
+!!! tip "Browser Display Tips"
+    - **Chrome/Edge**: JSON will be formatted automatically for easy reading
+    - **Firefox**: JSON appears with syntax highlighting
+    - **Safari**: Enable Develop menu → Show Page Source if needed
+    - **Other browsers**: If the browser downloads a file, open it with a text editor
+
+!!! warning "Token Security"
+    Keep your token private and do not share it. Anyone with your token can access your Zvuk Music account.
+
+### Settings
+
+- **Audio quality**: Select preferred audio quality
+    - `High (320 kbps)` - Available for all accounts (default)
+    - `Lossless (FLAC)` - Requires a Zvuk Music subscription
+
+## Known Issues / Notes
+
+### Authentication Issues
+
+- **Token expiration**: The token may expire and need to be refreshed periodically. If you encounter authentication errors, try obtaining a new token by following the steps above.
+- **Login required**: You must be logged in to zvuk.com before accessing the profile endpoint. If you see an error or empty response, make sure you're logged in to your account first.
+- **Invalid token format**: Ensure you copied the complete token value without any extra spaces, quotes, or line breaks.
+
+### Quality Issues
+
+- If lossless quality is unavailable (no subscription), the provider will automatically fall back to the highest available quality (320 kbps)
+
+## Not yet supported
+
+- Recommendations and personalized content
+- Lyrics display
+- Radio mode