fix(widget): apply native window fixes after frontend ready

[attson]

Root cause

The Linux frame-sync workaround ran once from Wails OnStartup. Wails invokes that callback concurrently with native window setup, before the GTK window is realized. gtk_widget_get_window() could therefore return NULL; the idle callback removed itself and the workaround never took effect. Under Mutter/X11 the widget could then remain alive while GTK stopped drawing and processing visible interaction after a frame-sync acknowledgement stalled.

Fix

• Keep required macOS OnStartup setup and repeat it idempotently from WidgetBridge.Ready().
• Keep Windows taskbar polling at startup and repeat it after Ready for delayed HWND registration.
• Move Linux frame-sync disabling to the post-frontend-ready hook, with a bounded 50ms retry window (10s max) so it waits for realization without creating a Wayland CPU loop.

Verification

• go test -tags webkit2_41 -timeout 60s ./desktop/
• go vet -tags webkit2_41 ./desktop/
• npm test -- --run (177 files, 2322 tests)
• GOOS=windows CGO_ENABLED=0 go test -c ...