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

static/locales/en.json

@@ -79,7 +79,13 @@
         "make_public": "Make public",
         "make_private": "Make private",
         "set_cover": "Set cover",
-        "cover_updated": "Cover updated"
+        "cover_updated": "Cover updated",
+        "search_audio": "Search audio files…",
+        "no_audio_files": "No audio files found",
+        "selected": "selected",
+        "loading": "Loading…",
+        "search_error": "Could not load audio files",
+        "adding": "Adding…"
     },
     "actions": {
         "search": "Search files...",