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/chunked-uploads.md

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+# Chunked Uploads
+
+OxiCloud supports TUS-like chunked uploads for large files. Uploads are parallel, resumable, and have MD5 integrity checks.
+
+## How It Works
+
+1. Client sends `POST /api/files/upload/init` with file metadata → receives an `upload_id`
+2. Client splits the file into chunks and uploads them in parallel via `POST /api/files/upload/chunk`
+3. Each chunk includes its index, MD5 hash, and the `upload_id`
+4. When all chunks are uploaded, client calls `POST /api/files/upload/complete`
+5. Server reassembles the file, verifies integrity, and runs deduplication
+
+## API Endpoints
+
+### Initialize Upload
+
+```http
+POST /api/files/upload/init
+Content-Type: application/json
+
+{
+  "file_name": "large-video.mp4",
+  "folder_id": "folder-uuid",
+  "total_size": 524288000,
+  "chunk_size": 8388608,
+  "total_chunks": 63
+}
+```
+
+### Upload Chunk
+
+```http
+POST /api/files/upload/chunk
+Content-Type: multipart/form-data
+
+upload_id: "uuid"
+chunk_index: 0
+chunk_hash: "md5-hex"
+file: <binary>
+```
+
+### Complete Upload
+
+```http
+POST /api/files/upload/complete
+Content-Type: application/json
+
+{
+  "upload_id": "uuid"
+}
+```
+
+## Configuration
+
+| Parameter | Default | Description |
+|---|---|---|
+| Max parallel chunks | 8 | Concurrent chunk uploads |
+| Min size for chunking | 200 MB | Below this, single-shot upload is used |
+| Chunk size | 8 MB | Default chunk size |
+
+## Frontend Behaviour
+
+The OxiCloud web UI automatically selects chunked upload for large files. A progress bar shows overall completion and current chunk status.