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/architecture/storage-quotas.md

@@ -0,0 +1,36 @@
+# Storage Quotas
+
+OxiCloud supports per-user storage quotas to limit disk usage.
+
+## Enabling Quotas
+
+```bash
+OXICLOUD_ENABLE_USER_STORAGE_QUOTAS=true
+```
+
+## How It Works
+
+1. Each user has a `storage_quota` field (in bytes, `0` = unlimited)
+2. On every file upload, the current usage is checked against the quota
+3. If the upload would exceed the quota, it's rejected with a `413 Payload Too Large` error
+4. Admins can view and set quotas via the admin panel or API
+
+## API
+
+| Method | Endpoint | Description |
+|--------|----------|-------------|
+| GET | `/api/admin/users/{id}/quota` | Get user's quota and current usage |
+| PUT | `/api/admin/users/{id}/quota` | Set user's quota |
+
+## Admin Panel
+
+The admin panel (`/admin.html`) shows each user's current usage vs. quota with a visual progress bar.
+
+## Deduplication Interaction
+
+Storage usage is calculated based on **logical file size** (what the user uploaded), not physical blob size. This means:
+
+- If two users upload the same 100 MB file, each user's quota is charged 100 MB
+- But on disk, only one 100 MB blob exists
+
+This ensures fair quota accounting while maintaining dedup benefits.