feat(client): add Tauri 2.x desktop client with sidecar process management
- Tauri 2.x project scaffold with dual-window (splash + main)
- Rust sidecar management: spawn/kill Python backend, port discovery via stdout
- CancellationToken for graceful task cancellation on exit
- System tray with show/quit, close-to-tray behavior
- Frontend: dynamic baseURL, SplashScreen, TitleBar, Tauri IPC adapter
- PyInstaller build scripts for cross-platform sidecar packaging
- GitHub Actions CI for Win/Mac/Linux release builds
- CSP security policy, proper capabilities configuration
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