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/.gitignore

@@ -7,12 +7,11 @@ node_modules/
 build/
 dist/
 
-# Generated WASM bundles. The committed `frontend/static/core.wasm`
-# (built by `make wasm` from `ui/wasm/`) is intentionally tracked so
-# Vitest and the SvelteKit dev server have the artefact available
-# without forcing every contributor to install TinyGo locally.
+# Generated WASM bundle (built by `make wasm` from `ui/wasm/`). No longer
+# committed: CI builds it (see `.gitea/actions/build-wasm`) and a local
+# dev build runs `make -C ui wasm` once (see `ui/docs/wasm-toolchain.md`).
 *.wasm
-!frontend/static/core.wasm
+frontend/static/wasm_exec.js
 
 # Wails desktop wrapper (Phase 31+)
 desktop/build/