Replace QWebEngineView-based start page with a rich native Qt panel.
QWebEngineView (PySide6.QtWebEngineWidgets) is not available on
conda-forge for Qt6, so the start page was always falling back to a
bare offline widget.
Start panel (silo_start.py):
- Database Items list with search from Silo REST API
- Recent Files 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 via QDesktopServices
- Catppuccin Mocha dark theme
kindred:// URL scheme:
- Desktop file registers x-scheme-handler/kindred MIME type
- MainWindow::processMessages() dispatches kindred:// URLs to Python
- handle_kindred_url() in silo_commands.py opens items by part number
- InitGui.py handles kindred:// URLs on cold start via QTimer
Closes#167
