Replace QWebEngineView-based start page with a rich native Qt panel that
fetches items directly from the Silo REST API. QWebEngineView is not
available on conda-forge for Qt6.
Start panel features:
- Database Items list with search (from SiloClient.list_items)
- Recent Files list from FreeCAD preferences
- Real-time Activity Feed via SSE (SiloEventListener)
- Context menu: Open in Create, Open in Browser, Copy Part Number
- Open in Browser button (QDesktopServices)
- Catppuccin Mocha dark theme styling
URL scheme support:
- handle_kindred_url() function for kindred://item/{part_number} URLs
- Startup hook in InitGui.py for cold-start URL arguments
Closes#167
