fix(gpu): guard against fatal wgpu device errors instead of panicking

Neither eframe nor egui-wgpu install a device-error/device-lost handler,
so a hybrid iGPU/dGPU switch invalidating the D3D12 device mid-session
crashed the process. Install callbacks that flag the loss and relaunch
the app cleanly via ViewportCommand::Close instead. The wallpaper host
just closes on loss and lets the main app's supervisor respawn it.

Closes #176