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/main.js

@@ -7,6 +7,7 @@ window.App = window.App || {};
         App.ui.applyTheme();
         App.ui.applyPreferredQuality();
         App.ui.applyFeedEndBehavior();
+        App.ui.applyDensity();
         App.ui.renderMenu();
         App.favorites.renderBar();
         App.ui.bindGlobalHandlers();