ci: add Prettier format check GitHub Action

Adds a format-check workflow that runs `pnpm run format:check` (Lerna fans
out to all packages with the script) on pushes to main/develop and all PRs.
Also adds the missing `format:check` script to the server package so Lerna
picks it up alongside the existing webapp script.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/format-check.yml

@@ -0,0 +1,65 @@
+name: Format Check
+
+on:
+  push:
+    branches:
+      - main
+      - develop
+    paths:
+      - '**.ts'
+      - '**.tsx'
+      - '**.js'
+      - '**.jsx'
+      - '**.json'
+      - '**.md'
+      - '**/.prettierrc'
+      - '**/prettier.config.*'
+      - 'pnpm-lock.yaml'
+      - 'package.json'
+      - 'packages/*/package.json'
+      - '.github/workflows/format-check.yml'
+  pull_request:
+    paths:
+      - '**.ts'
+      - '**.tsx'
+      - '**.js'
+      - '**.jsx'
+      - '**.json'
+      - '**.md'
+      - '**/.prettierrc'
+      - '**/prettier.config.*'
+      - 'pnpm-lock.yaml'
+      - 'package.json'
+      - 'packages/*/package.json'
+      - '.github/workflows/format-check.yml'
+
+defaults:
+  run:
+    shell: 'bash'
+
+jobs:
+  format-check:
+    name: Prettier Format Check
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Install pnpm
+        uses: pnpm/action-setup@v4
+        with:
+          version: 9
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: '18'
+          cache: 'pnpm'
+
+      - name: Install dependencies
+        run: pnpm install --frozen-lockfile
+
+      - name: Run format check
+        run: pnpm run format:check