Replace video player with a fully custom fake-fullscreen HUD

Single fullscreen player state everywhere (desktop/Android/iOS/feed) instead
of the old modal-vs-native-fullscreen split, with custom controls: draggable
timeline with buffered range, dynamically-escalating skip buttons (double-tap
zones too), per-video format switching, favorites, PiP with auto-PiP on
backgrounding, volume swipe, TikTok-style HUD auto-hide, and swipe-down/
back-button/close-button dismissal. Reels feed reuses the same skip/format/
PiP logic via the new customPlayer.js shared module.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/js/state.js

@@ -10,10 +10,6 @@ App.state = {
     hlsPlayer: null,
     currentLoadController: null,
     errorToastTimer: null,
-    playerMode: 'modal',
-    playerHome: null,
-    onFullscreenChange: null,
-    onWebkitEndFullscreen: null,
     loadedVideos: [],
     feedOpen: false,
     feedMuted: true,