web: the app's type-check leaves the unit tests out; CI checks them separately

Run 8478 passed every test lane and then failed `Build & push image`. The
Dockerfile's frontend stage copies only frontend/, and `npm run build`
type-checks with tsconfig.json, which covered grammar.test.ts. Its import of
../core/testdata/grammar.json doesn't exist inside that stage. Nothing was
published: the build is the publish and it stopped.

tsconfig.json now excludes `*.test.ts`. The new tsconfig.test.json extends it
with the tests included, and ci.yml's typecheck lane runs that one, so the
tests are still type-checked before anything ships.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/tsconfig.test.json

@@ -0,0 +1,9 @@
+{
+  // The app's config plus its unit tests. tsconfig.json leaves the tests out because
+  // the production build type-checks in a Docker stage that copies only frontend/,
+  // and grammar.test.ts reads ../core/testdata, which is not there (run 8478).
+  // CI's typecheck lane runs THIS file, so the tests are still type-checked.
+  "extends": "./tsconfig.json",
+  "include": ["src"],
+  "exclude": []
+}