feat: marketplace integration via backend proxy

- market_url config (server-side only, never exposed to frontend)
- /api/market/* proxy endpoints to PocketBase
- Dashboard page: 'Market' tab in sub-sidebar
- Market list: browse/install community dashboards
- Login/register modal, upload to market
- PocketBase API fully proxied, zero credentials in frontend

requirements.txt

@@ -4,3 +4,4 @@ websockets==14.1
 aiofiles==24.1.0
 jinja2==3.1.4
 python-multipart==0.0.18
+httpx==0.28.1