feat: add VitePress docs site, music file picker modal, Dockerfile optimization, i18n keys for 14 locales

- Add docs/ with VitePress site (19 pages): guide, config, architecture, FAQ
- Add GitHub Actions workflow for auto-deploy to GitHub Pages
- Replace music 'Add Tracks' upload picker with in-app audio file browser modal
- Add music picker CSS styles with dark theme support
- Add missing i18n keys (search_audio, no_audio_files, etc.) to all 14 locales
- Optimize Dockerfile: shared base stage, COPY --chmod, consolidated RUN, HEALTHCHECK
- Improve README: docs links, updated stats (222+ tests, 14 languages), feature status

docs/package.json

@@ -0,0 +1,10 @@
+{
+  "scripts": {
+    "docs:dev": "vitepress dev .",
+    "docs:build": "vitepress build .",
+    "docs:preview": "vitepress preview ."
+  },
+  "devDependencies": {
+    "vitepress": "^1.6.3"
+  }
+}