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>

Dockerfile

@@ -1,5 +1,12 @@
+FROM node:20-alpine AS builder
+WORKDIR /app
+COPY package.json ./
+RUN npm install
+COPY . .
+RUN npm run build
+
 FROM nginx:alpine
-COPY index.html styles.css app.js sw.js manifest.json icon.svg /usr/share/nginx/html/
+COPY --from=builder /app/dist /usr/share/nginx/html/
 COPY nginx.conf /etc/nginx/conf.d/default.conf
 COPY docker-entrypoint.sh /docker-entrypoint.sh
 RUN chmod +x /docker-entrypoint.sh