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/nl.json

@@ -62,7 +62,13 @@
         "add_error": "Kon tracks niet toevoegen aan playlist",
         "no_playlists_yet": "Nog geen playlists. Maak er eerst een!",
         "selected_files": "Geselecteerd:",
-        "error": "Fout"
+        "error": "Fout",
+        "search_audio": "Audiobestanden zoeken…",
+        "no_audio_files": "Geen audiobestanden gevonden",
+        "selected": "geselecteerd",
+        "loading": "Laden…",
+        "search_error": "Kan audiobestanden niet laden",
+        "adding": "Toevoegen…"
     },
     "actions": {
         "search": "Zoek bestanden...",