ci: typecheck the GUI in the gui-unit job

The GUI README tells contributors to run `npx tsc --noEmit` and `npm run build`
starts with `tsc`, but CI never runs either — a type error ships green today
and only surfaces when someone next builds a bundle locally. Add the check as
one step inside the existing gui-unit job so it reuses that job's npm ci and
costs no extra runner.

Verified locally: `npx tsc --noEmit` exits 0 on current main, and the vitest
suite (82 tests) still passes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -32,6 +32,9 @@ jobs:
       - name: npm ci
         working-directory: surfaces/gui
         run: npm ci
+      - name: Typecheck
+        working-directory: surfaces/gui
+        run: npx tsc --noEmit
       - name: Unit tests
         working-directory: surfaces/gui
         run: npm test