Migrate to Vite + ES module architecture

Split monolithic app.js (~1850 lines) into maintainable ES modules:
- src/state.js  — shared mutable state (S object)
- src/utils.js  — md5, esc, formatDur, toast
- src/api.js    — Subsonic API, Navidrome REST, iTunes API
- src/player.js — playback, transport, queue, media session, preview
- src/views.js  — navigation, page loaders, card makers, search
- src/ui.js     — all modals, overlays, sidebar, settings
- src/pwa.js    — service worker registration + update banner
- src/main.js   — app init, connect, window globals

Updated Dockerfile to multi-stage (Node build → nginx serve).
Static assets (sw.js, manifest.json, icon.svg) moved to public/.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/state.js

@@ -0,0 +1,47 @@
+// Shared mutable state — imported as { S } in every module
+export const S = {
+  cfg: { url: '', user: '', pass: '', lastfmKey: '', backendUrl: '' },
+
+  // Playback
+  queue: [],
+  qIdx: -1,
+  shuffle: false,
+  repeat: false,
+  radioMode: false,
+  audio: null,
+  previewAudio: null,
+  previewCurrent: null,
+  _scrubbing: false,
+  _msPosLast: 0,
+  queueOpen: false,
+
+  // Now Playing overlay
+  npOpen: false,
+  npQueueMode: false,
+
+  // Navigation history
+  _navStack: [],
+  _navCursor: -1,
+  _navJumping: false,
+
+  // Search
+  searchTimer: null,
+  globalSearchTimer: null,
+  _mobileSearchTimer: null,
+
+  // Download
+  downloadTarget: null,
+  dlType: 'track',
+  jobs: {},
+
+  // Playlists
+  _currentPlaylist: null,
+  plModalSongs: [],
+  cachedPlaylists: [],
+
+  // Navidrome REST auth
+  _ndToken: null,
+
+  // Artist images cache
+  artistImgCache: {},
+};