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/pwa.js

@@ -0,0 +1,34 @@
+export function injectPWA() {
+  if(!('serviceWorker' in navigator)) return;
+
+  navigator.serviceWorker.addEventListener('message', e => {
+    if(e.data?.type === 'SW_UPDATED') {
+      localStorage.setItem('sw-version', e.data.version);
+      showUpdateBanner(e.data.version);
+    }
+    if(e.data?.type === 'VERSION') {
+      const prev = localStorage.getItem('sw-version');
+      if(prev && prev !== e.data.version) showUpdateBanner(e.data.version);
+      localStorage.setItem('sw-version', e.data.version);
+    }
+  });
+
+  navigator.serviceWorker.register('sw.js', { updateViaCache: 'none' }).then(reg => {
+    reg.update().catch(() => {});
+    const ctrl = navigator.serviceWorker.controller;
+    if(ctrl) ctrl.postMessage({ type: 'GET_VERSION' });
+  }).catch(() => {});
+}
+
+export function showUpdateBanner(version) {
+  if(document.getElementById('update-banner')) return;
+  const b = document.createElement('div');
+  b.id = 'update-banner';
+  b.className = 'update-banner';
+  b.innerHTML = `
+    <span class="update-banner-text">✦ Update verfügbar${version ? ' <span style="opacity:.55;font-size:10px">('+version+')</span>' : ''}</span>
+    <button class="update-banner-btn" onclick="window.location.reload()">Neu laden</button>
+    <button class="update-banner-close" onclick="document.getElementById('update-banner').remove()">✕</button>
+  `;
+  document.body.appendChild(b);
+}