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/webdav.md

@@ -0,0 +1,80 @@
+# WebDAV
+
+OxiCloud exposes a fully RFC 4918 compliant WebDAV interface at `/webdav/`. It works with all major file managers and sync clients.
+
+## Base URL
+
+```
+https://your-server:8086/webdav/
+```
+
+## Authentication
+
+HTTP Basic Authentication:
+
+```
+Authorization: Basic base64(username:password)
+```
+
+::: tip
+Always use HTTPS in production — Basic auth sends credentials in every request.
+:::
+
+## Supported Methods
+
+| Method | Description |
+|--------|-------------|
+| `PROPFIND` | List directory contents / get file properties |
+| `GET` | Download a file |
+| `PUT` | Upload a file |
+| `MKCOL` | Create a folder |
+| `MOVE` | Move or rename a file/folder |
+| `COPY` | Copy a file/folder |
+| `DELETE` | Delete a file/folder |
+| `LOCK` / `UNLOCK` | File locking |
+
+## Client Setup
+
+### Windows Explorer
+
+1. Open **This PC** → **Map network drive**
+2. Enter: `https://your-server:8086/webdav/`
+3. Check **Connect using different credentials**
+4. Enter your OxiCloud username and password
+
+### macOS Finder
+
+1. **Go** → **Connect to Server** (⌘K)
+2. Enter: `https://your-server:8086/webdav/`
+3. Enter credentials when prompted
+
+### Linux (Nautilus / Files)
+
+1. Open Files → **Other Locations**
+2. In the address bar, type: `davs://your-server:8086/webdav/`
+3. Enter credentials
+
+### Linux (Dolphin / KDE)
+
+1. In the address bar, type: `webdavs://your-server:8086/webdav/`
+
+### Command Line (curl)
+
+```bash
+# List root directory
+curl -u user:pass -X PROPFIND https://your-server:8086/webdav/ \
+  -H "Depth: 1"
+
+# Download a file
+curl -u user:pass https://your-server:8086/webdav/document.pdf -o document.pdf
+
+# Upload a file
+curl -u user:pass -T localfile.txt https://your-server:8086/webdav/remotefile.txt
+
+# Create a folder
+curl -u user:pass -X MKCOL https://your-server:8086/webdav/new-folder/
+```
+
+## Streaming PROPFIND
+
+OxiCloud streams PROPFIND responses, so listing directories with thousands of files doesn't consume excessive memory.