chore: use npm ci instead of npm install in CI

- npm ci is faster than npm install in CI environments
- Strictly enforces package-lock.json consistency
- Removes node_modules before installing (clean slate)
- Fails if package.json and package-lock.json are out of sync
- Aligns with deploy-worker.yml which already uses npm ci

Author: claude

.github/workflows/ci.yml

@@ -141,7 +141,7 @@ jobs:
           path: build/web
 
       - name: Install npm dependencies
-        run: npm install
+        run: npm ci
 
       - name: Install Playwright browsers
         run: npx playwright install --with-deps chromium