build(ui): build core.wasm in CI, stop committing the binary (F6)

core.wasm and wasm_exec.js are no longer tracked (untracked + gitignored).
A reusable composite action .gitea/actions/build-wasm installs TinyGo
(actions/cache'd) and runs `make -C ui wasm`; it runs in all three
frontend-building workflows — ui-test (before Playwright; Vitest uses the
fake Core and needs no build), dev-deploy, and prod-build. ui-test gains a
Go setup (TinyGo shells out to Go); the deploy workflows already had one.

Docs: ui/docs/wasm-toolchain.md, ui/README.md.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ui/README.md

@@ -81,7 +81,7 @@ ui/
     ├── src/platform/store/  KeyStore/Cache interfaces + web adapter
     ├── src/proto/           generated Protobuf-ES + Connect descriptors + FlatBuffers TS bindings
     ├── src/routes/          SvelteKit routes (/, /login, /lobby, /lobby/create)
-    └── static/              core.wasm + wasm_exec.js (committed artefacts)
+    └── static/              core.wasm + wasm_exec.js (built by `make wasm` / CI; gitignored)
 ```
 
 Linked topic docs: