feat: use FontAwesome arrow icons for back/forward nav buttons

Replace the sprite-icon `leftArrow` / `rightArrow` / `leftArrowDisabled`
/ `rightArrowDisabled` bitmaps with fa-arrow-left / fa-arrow-right FA
icons, matching the weight of the adjacent FA-based sidebar and
toolbar buttons (binoculars, search, design-mode toggle) and giving
the back/forward pair a browser-style affordance.

NavigationProvider still toggles `.nav-back-btn` ↔ `.nav-back-btn-disabled`
(and the forward pair) to signal enabled state. The CSS is rewritten as
a single icon-button block with a .disabled opacity fork instead of
four separate sprite rules.

Author: abose

src/index.html

@@ -914,8 +914,8 @@
                 <a id="phcode-io-main-nav" href="https://phcode.io" target="_blank" rel="noopener">phcode.io</a>
             </div>
             <div id="mainNavBarRight">
-                <div id="navBackButton" class="nav-back-btn btn-alt-quiet"></div>
-                <div id="navForwardButton" class="nav-forward-btn btn-alt-quiet"></div>
+                <div id="navBackButton" class="nav-back-btn btn-alt-quiet"><i class="fa-solid fa-arrow-left"></i></div>
+                <div id="navForwardButton" class="nav-forward-btn btn-alt-quiet"><i class="fa-solid fa-arrow-right"></i></div>
                 <div id="showInfileTree" class="show-in-file-tree-btn btn-alt-quiet"></div>
                 <div id="searchNav" class="search-nav-btn btn-alt-quiet"></div>
                 <div class="working-set-splitview-btn btn-alt-quiet"></div>

src/styles/brackets.less

@@ -1295,44 +1295,41 @@ a, img {
     z-index: 1;
 }
 
-.nav-back-btn {
+/* Back / forward nav buttons use FontAwesome arrows so they match the
+   browser's back/forward affordance and stay visually consistent with
+   the other FA-based sidebar and toolbar buttons. NavigationProvider
+   still toggles .nav-back-btn / .nav-back-btn-disabled (and the forward
+   pair) to signal the enabled state — the rules below read as a single
+   icon button with an enabled/disabled fork. */
+.nav-back-btn,
+.nav-forward-btn,
+.nav-back-btn-disabled,
+.nav-forward-btn-disabled {
     top: 7px;
     padding: 4px 6px;
-    .sprite-icon(0, 0, 13px, 13px, "images/sprites.svg#leftArrow");
-    background-position: center;
-    background-size: cover;
-    opacity: 0.8;
+    width: 13px;
+    height: 13px;
+    display: inline-flex;
+    align-items: center;
+    justify-content: center;
+    font-size: 12px;
+    line-height: 1;
     z-index: 1;
-}
 
-.nav-forward-btn {
-    top: 7px;
-    padding: 4px 6px;
-    .sprite-icon(0, 0, 13px, 13px, "images/sprites.svg#rightArrow");
-    background-position: center;
-    background-size: cover;
-    opacity: 0.8;
-    z-index: 1;
+    i {
+        pointer-events: none;
+    }
 }
 
-.nav-back-btn-disabled {
-    top: 7px;
-    padding: 4px 6px;
-    .sprite-icon(0, 0, 13px, 13px, "images/sprites.svg#leftArrowDisabled");
-    background-position: center;
-    background-size: cover;
+.nav-back-btn,
+.nav-forward-btn {
     opacity: 0.8;
-    z-index: 1;
 }
 
+.nav-back-btn-disabled,
 .nav-forward-btn-disabled {
-    top: 7px;
-    padding: 4px 6px;
-    .sprite-icon(0, 0, 13px, 13px, "images/sprites.svg#rightArrowDisabled");
-    background-position: center;
-    background-size: cover;
-    opacity: 0.8;
-    z-index: 1;
+    opacity: 0.35;
+    pointer-events: none;
 }
 
 .splitview-icon-none {