ci: install PHP/composer in build-assets for wayfinder

Vite Wayfinder plugin invokes 'php artisan wayfinder:generate' during
build, which needs vendor/autoload.php. Add Setup PHP + composer install
(with cache) before bun run build.

.github/workflows/ci.yml

@@ -23,6 +23,27 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: 8.4
+          tools: composer:v2
+          coverage: none
+
+      - name: Get composer cache directory
+        id: composer-cache
+        run: echo "dir=$(composer config cache-files-dir)" >> "$GITHUB_OUTPUT"
+
+      - name: Cache composer dependencies
+        uses: actions/cache@v4
+        with:
+          path: ${{ steps.composer-cache.outputs.dir }}
+          key: composer-${{ runner.os }}-php8.4-${{ hashFiles('composer.lock') }}
+          restore-keys: composer-${{ runner.os }}-php8.4-
+
+      - name: Install PHP Dependencies
+        run: composer install --no-interaction --prefer-dist --optimize-autoloader
+
       - name: Setup Bun
         uses: oven-sh/setup-bun@v2