chore: add VS Code workspace config and improve CI infrastructure

Add .vscode/ with Biome formatter, Tailwind, Vitest, Playwright, and
Python debug configs. Extract shared pnpm/Node setup into a composite
GitHub Action and add Dependabot and dependency-review workflows.

.github/actions/setup/action.yml

@@ -0,0 +1,15 @@
+name: Setup Project
+description: Install pnpm, Node.js 22, and dependencies
+
+runs:
+  using: composite
+  steps:
+    - uses: pnpm/action-setup@v4
+
+    - uses: actions/setup-node@v4
+      with:
+        node-version: 22
+        cache: pnpm
+
+    - run: pnpm install --frozen-lockfile
+      shell: bash