ci(client): hold strict null check errors per file to a baseline that only shrinks

The client compiles with strict: false, so nullable contract fields arrive as
if always present and implicit anys stay invisible. tsconfig.strict.json is the
same program with strictNullChecks and noImplicitAny, and lint:strict
(scripts/strict-lint.mjs) counts its errors per file against
scripts/strict-baseline.json: a file may not gain one, a new file has to be
clean, and an entry above its count fails until --update lowers it. Nothing is
converted here; today's 534 errors in 165 files are frozen. allowJs stays: the
tests import the .mjs scripts and read their JSDoc types through it.
The todo MSW handler maps a cleared priority to its default so the test
helper written in this branch starts at zero. The check runs after the
typecheck in client-quality.

.github/workflows/test.yml

@@ -380,6 +380,13 @@ jobs:
       - name: Typecheck
         run: cd client && npm run typecheck
 
+      - name: Strict type check
+        # tsconfig.strict.json is the same program with strictNullChecks and
+        # noImplicitAny; its errors per file may not rise above
+        # scripts/strict-baseline.json, and a file without an entry, every new
+        # one included, has to be clean (scripts/strict-lint.mjs).
+        run: cd client && npm run lint:strict
+
       - name: Lint
         # Fails on any ESLint error, like lint:check did, and on any rule
         # whose warning count rises above scripts/eslint-baseline.json.