Fix CI: pin Node 24 and install frontend deps with npm ci

CI ran `yarn && yarn dev` with no committed yarn.lock, so every run resolved
the latest matching versions and dependency drift broke the pipeline in two
independent ways:

- select2 4.1.0 added engines.node ">=24" but the runner used node 22, so
  `yarn install` failed outright on every pull request.
- webpack 5.108 removed lib/SizeFormatHelpers, which laravel-mix 6 still
  requires, so `yarn dev` would have failed the build regardless of node.

Switch the workflow to `npm ci`, which installs the exact, known-good
versions already pinned in package-lock.json (select2 4.0.13, webpack
5.100.1) and is verified to build and lint cleanly. Pin the runner to node
24 via actions/setup-node so the toolchain is explicit rather than tracking
the runner default. Also pin select2 to ~4.0.13 in package.json so a future
`npm install` cannot pull the incompatible 4.1.0 back in.

.github/workflows/ci.yml

@@ -31,17 +31,17 @@ jobs:
           cp .env.example .env
           php artisan key:generate
 
-      - name: Cache yarn dependencies
-        uses: actions/cache@v4
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
         with:
-          path: node_modules
-          key: yarn-${{ hashFiles('yarn.lock') }}
+          node-version: '24'
+          cache: 'npm'
 
-      - name: Run yarn
-        run: yarn && yarn dev
+      - name: Install node modules and build assets
+        run: npm ci && npm run dev
 
       - name: Run ESLint
-        run: yarn lint
+        run: npm run lint
 
       - name: Run tests
         run: php artisan test