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>

.gitea/workflows/prod-build.yaml

@@ -87,6 +87,9 @@ jobs:
         working-directory: ui
         run: pnpm install --frozen-lockfile
 
+      - name: Build core.wasm
+        uses: ./.gitea/actions/build-wasm
+
       - name: Build UI bundle
         working-directory: ui/frontend
         env: