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/fr.json

@@ -62,7 +62,13 @@
         "add_error": "Impossible d'ajouter les pistes",
         "no_playlists_yet": "Pas encore de playlists. Créez-en une d'abord !",
         "selected_files": "Sélectionnés :",
-        "error": "Erreur"
+        "error": "Erreur",
+        "search_audio": "Rechercher des fichiers audio…",
+        "no_audio_files": "Aucun fichier audio trouvé",
+        "selected": "sélectionnés",
+        "loading": "Chargement…",
+        "search_error": "Impossible de charger les fichiers audio",
+        "adding": "Ajout en cours…"
     },
     "actions": {
         "search": "Rechercher des fichiers...",