web: FieldCraft in the browser (apps/fieldcraft-web)

The desktop's ModelApp in a web page through eframe's web runner
(WebGPU, WebGL2 fallback, `?webgl` to force it), built with Trunk.
The browser services sit behind the UI's host-service seams: File >
Open uses the browser's file picker, dropped files are read
asynchronously, and both go into the page's files; Save, Save As and
Write Deck hand the written file out as a download. Autosaves go to the
recovery folder as on the desktop and are mirrored to local storage, so
File > Recover offers what a closed or crashed page left. `?sample=NAME`
opens a sample. A loading screen with the app icon, and a message
instead of the app when neither WebGPU nor WebGL2 is available.

`cargo xtask wasm` (and so `cargo xtask ci`) now checks fieldcraft-web
for wasm32 with the library crates. docs/web.md: running it locally,
what works in the browser and what does not.

Part of #119

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

apps/fieldcraft-web/Trunk.toml

@@ -0,0 +1,10 @@
+# Build with: trunk build --release   (from apps/fieldcraft-web; docs/web.md)
+[build]
+target = "index.html"
+dist = "../../dist/web"
+public_url = "./"
+filehash = true
+
+[serve]
+addresses = ["127.0.0.1"]
+port = 8790