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>

public/sw.js

@@ -0,0 +1,40 @@
+const CACHE = 'ns-v38';
+const PRECACHE = ['/', 'index.html', 'manifest.json', 'icon.svg'];
+const SKIP_PATHS = ['/rest/stream', '/rest/download', '/config.js'];
+
+self.addEventListener('install', e => {
+  e.waitUntil(caches.open(CACHE).then(c => c.addAll(PRECACHE)));
+  self.skipWaiting();
+});
+
+self.addEventListener('activate', e => {
+  e.waitUntil(
+    caches.keys()
+      .then(keys => Promise.all(keys.filter(k => k !== CACHE).map(k => caches.delete(k))))
+      .then(() => self.clients.claim())
+      .then(() => self.clients.matchAll({ type: 'window', includeUncontrolled: true }))
+      .then(clients => clients.forEach(c => c.postMessage({ type: 'SW_UPDATED', version: CACHE })))
+  );
+});
+
+self.addEventListener('message', e => {
+  if (e.data?.type === 'GET_VERSION') {
+    e.source.postMessage({ type: 'VERSION', version: CACHE });
+  }
+});
+
+self.addEventListener('fetch', e => {
+  if (!e.request.url.startsWith(self.location.origin)) return;
+  if (SKIP_PATHS.some(p => e.request.url.includes(p))) return;
+  e.respondWith(
+    fetch(e.request)
+      .then(res => {
+        if (res.status === 200 && e.request.method === 'GET') {
+          const clone = res.clone();
+          caches.open(CACHE).then(c => c.put(e.request, clone));
+        }
+        return res;
+      })
+      .catch(() => caches.match(e.request).then(r => r || Response.error()))
+  );
+});