WebGPU device loss handling and UI synchronization

Losing the already initialized main browser UI WebGPU device freezes the editor on its last frame. Subsequent edits still succeed and CPU frame rendering returns the updated pixels, so the display silently diverges from the composition.

## Environment

- Fresh browser release build of current main `e38a92e792c07f1c57c5dcfcab37d91932b9e2b8`, checked 2026-10-07; Rust 1.95.0 and wasm-bindgen 0.2.129.
- macOS 26.5.2 arm64, Apple M1; isolated Chrome 154.0.8037.93 and Playwright WebKit 26.6, headless. WebKit is not physical Safari.
- `?empty&storage=memory&noworkers&nosw`; explicitly choose CPU composition rendering. Actual app info reports BrowserWebGpu presentation, zero frame workers, zero job workers and no GPU compositor.

## Controlled reproduction

1. Before startup, wrap `GPUAdapter.prototype.requestDevice` to record and return the real device without changing its configuration. Observe its real `lost` promise.
2. Create a 128×128 one-second composition with a blue solid. Use `render.backend {"backend":"cpu"}`. Confirm blue in the visible Composition panel.
3. Change the solid to red with `layer.settings {"color":"#ff0000"}`. Confirm the visible red image and independently decode the PNG returned by `render.frame {"max_side":0,"base64":true}`: all 16,384 pixels are red. Undo and confirm the visible blue image.
4. Call `destroy()` on the single recorded real main UI GPUDevice. Require its lost promise to report `reason:"destroyed"`. No worker or compositor device is present...