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/guide/search.md

@@ -0,0 +1,43 @@
+# Search
+
+OxiCloud provides full-text search across your files with multiple filter options.
+
+## Endpoint
+
+```http
+GET /api/search?q=report&type_filter=pdf,docx&recursive=true
+```
+
+## Query Parameters
+
+| Parameter | Description |
+|---|---|
+| `q` | Search query (matches file name) |
+| `type_filter` | Comma-separated file extensions to filter by |
+| `folder_id` | Restrict search to a specific folder |
+| `recursive` | `true` to search subfolders |
+| `date_from` / `date_to` | Filter by modification date |
+| `size_min` / `size_max` | Filter by file size (bytes) |
+| `limit` | Maximum results to return |
+| `offset` | Pagination offset |
+
+## How It Works
+
+OxiCloud stores file metadata in PostgreSQL with an `ltree` path column, enabling efficient recursive subtree queries:
+
+```sql
+SELECT * FROM storage.files
+WHERE path <@ 'root.folder_id'
+  AND LOWER(name) LIKE '%query%'
+  AND LOWER(extension) = ANY('{pdf,docx}')
+ORDER BY updated_at DESC
+LIMIT 50;
+```
+
+## Frontend
+
+The web UI includes a search bar in the toolbar. Results appear instantly with file name, path, size, and type. Clicking a result navigates to the file's location.
+
+## Feature Flag
+
+Search can be disabled via `OXICLOUD_ENABLE_SEARCH=false`.