Web: a lost graphics device says so and offers a reload (#225)

When the page's WebGPU device was lost, the canvas kept showing its
last frame: edits still went through and rendered frames were right,
but the editor never showed them and nothing said why (the status
message about it is drawn in the canvas that can't present any more).
The page now covers the canvas with a notice that offers Save Project
(a download) and Reload. Reload first writes pending changes to
browser storage, so the session snapshot brings the project and its
unsaved changes back; if that write fails it says so and offers
Reload Anyway. Without a kept session (?empty, memory storage) the
notice says to save first.

Test: apps/effectcraft-web/tests/page.mjs (Node, fake DOM).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/web.md

@@ -263,6 +263,13 @@ eyedroppers read GPU frames back asynchronously (`Gpu::read_display_async`: the
 frame later). On WebGL2, or without a usable adapter, everything renders on the CPU.
 `effectcraft.info().gpu` reports `{compositor, viewerOnGpu}`.
 
+If the page's WebGPU device is lost (a driver reset, the browser reclaiming the GPU), the canvas
+can't show anything new, while the project stays open and commands keep working. The page then
+covers the canvas with a notice (`showDeviceLost` in `js/host.js`) offering **Save Project** (a
+download) and **Reload**, which first writes pending changes to browser storage
+(`effectcraft.flush()`) so the session comes back after the reload; when the write fails it says
+so and offers **Reload Anyway**.
+
 **GPU effects run in the frame workers.** A browser never lets JavaScript (or wasm) wait for a GPU
 readback, not even in a worker: a buffer maps only once the thread returns to its event loop.
 The CPU renderer, which runs GPU effect chains and reads their results back mid-frame, therefore
@@ -420,7 +427,8 @@ reused worker again even at the same size, and a worker that fails to start fail
 terminated (`node apps/effectcraft-web/tests/workers.mjs`).
 
 `apps/effectcraft-web/tests/page.mjs` checks page-side helpers of `js/host.js` the same way with
-a fake DOM: a picked file that can't be read is reported while the others are added
+a fake DOM: a picked file that can't be read is reported while the others are added, and the
+device-lost notice saves, flushes before reloading and reports a failed flush
 (`node apps/effectcraft-web/tests/page.mjs`).
 
 It also checks the M13.10 paths: viewer frames rendered in frame workers while scrubbing with the