chore: pin prettier to 3.9.5 and run format check after install (#1716)

The "Check formatting" step in main.yml ran `npx prettier --check .`
before `npm install`, so npx fetched the LATEST prettier rather than the
version pinned in package-lock.json. This made the format check
non-deterministic and red on open PRs whenever the latest published
prettier differed from the pinned one.

- Pin prettier to exact 3.9.5 in devDependencies and update the lockfile
- Reformat the repo with the pinned prettier (expected source churn)
- Move the format check after `npm install --no-package-lock` and run it
  via `npm run prettier-check` so CI uses the locally-installed pinned
  prettier


Claude-Session: https://claude.ai/code/session_01HiccCEh9mwCfVzE8qYcop1

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.github/workflows/main.yml

@@ -14,9 +14,6 @@ jobs:
     steps:
       - uses: actions/checkout@v6
 
-      - name: Check formatting
-        run: npx prettier --check .
-
       - uses: actions/setup-node@v6
         with:
           node-version-file: package.json
@@ -26,6 +23,9 @@ jobs:
       # - run: npm ci
       - run: npm install --no-package-lock
 
+      - name: Check formatting
+        run: npm run prettier-check
+
       - name: Check version consistency
         run: npm run check-version