chore: add root npm workspace and update gitignore

- Root package.json with npm run dev (concurrently API + Vite)
- Add node_modules/, frontend/dist/ to .gitignore
- Whitelist package.json and package-lock.json

package.json

@@ -0,0 +1,10 @@
+{
+  "name": "bitcoin-address-generator",
+  "private": true,
+  "scripts": {
+    "dev": "concurrently --kill-others -n API,Vite \"venv/bin/python -m uvicorn src.api:app --port 8000\" \"npm --prefix frontend run vite\""
+  },
+  "devDependencies": {
+    "concurrently": "^9.2.1"
+  }
+}