Add modern UI enhancements over the classic redesign

Layered progressive polish on the warm classic + brass theme:
- Card entrance animation on first mount (virtualizer-aware)
- Cursor-tracking brass spotlight border on cards
- Thumbnail skeleton shimmer until the poster paints
- Hover video preview after a short dwell (only when formats resolved)
- View Transition + blurred-poster ambient backdrop on player open
- Favorite heart pop + expanding ring on add
- ⌘K command palette (search, theme, density, reels, source/channel)
- Scroll-progress bar + back-to-top FAB
- Grid density toggle (comfortable/compact)
- Reels HUD: serif title, brass scrubber, muted-state pulse

All new motion respects prefers-reduced-motion; no JS/HTML structure
changes to the core grid/feed. New glue lives in frontend/js/enhance.js.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/js/ui.js

@@ -21,6 +21,13 @@ App.ui = App.ui || {};
         if (select) select.value = App.storage.getFeedEndBehavior();
     };
 
+    App.ui.applyDensity = function() {
+        const density = App.storage.getDensity();
+        document.body.dataset.density = density;
+        const select = document.getElementById('density-select');
+        if (select) select.value = density;
+    };
+
     // Toast helper for playback + network errors.
     App.ui.showError = function(message) {
         const toast = document.getElementById('error-toast');
@@ -286,6 +293,18 @@ App.ui = App.ui || {};
             };
         }
 
+        const densitySelect = document.getElementById('density-select');
+        if (densitySelect) {
+            densitySelect.value = App.storage.getDensity();
+            densitySelect.onchange = () => {
+                App.storage.setDensity(densitySelect.value);
+                App.ui.applyDensity();
+                if (App.virtualGrid && typeof App.virtualGrid.relayout === 'function') {
+                    App.virtualGrid.relayout();
+                }
+            };
+        }
+
         const feedEndSelect = document.getElementById('feed-end-select');
         if (feedEndSelect) {
             feedEndSelect.value = App.storage.getFeedEndBehavior();