ci: add prettier auto-format workflow

.github/workflows/format.yml

@@ -0,0 +1,66 @@
+name: Format
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+concurrency:
+  group: format-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: write
+
+jobs:
+  format-check:
+    name: Prettier
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          ref: ${{ github.head_ref || github.ref_name }}
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: 20
+
+      - name: Setup pnpm
+        uses: pnpm/action-setup@v4
+        with:
+          version: 9
+
+      - name: Get pnpm store directory
+        id: pnpm-cache
+        shell: bash
+        run: echo "STORE_PATH=$(pnpm store path)" >> "$GITHUB_OUTPUT"
+
+      - name: Cache pnpm dependencies
+        uses: actions/cache@v4
+        with:
+          path: ${{ steps.pnpm-cache.outputs.STORE_PATH }}
+          key: pnpm-store-${{ runner.os }}-${{ hashFiles('pnpm-lock.yaml') }}
+          restore-keys: |
+            pnpm-store-${{ runner.os }}-
+
+      - name: Install dependencies
+        run: pnpm install --frozen-lockfile
+
+      - name: Check formatting (PR)
+        if: github.event_name == 'pull_request'
+        run: pnpm prettier --check "src/**/*.{ts,tsx,css}" "*.{json,js,cjs,ts,md}"
+
+      - name: Fix formatting (push to main)
+        if: github.event_name == 'push'
+        run: pnpm format
+
+      - name: Commit formatting fixes
+        if: github.event_name == 'push'
+        uses: stefanzweifel/git-auto-commit-action@v5
+        with:
+          commit_message: "style: auto-format with prettier"
+          file_pattern: "src/**/*.ts src/**/*.tsx src/**/*.css *.json *.js *.cjs *.ts *.md"