chore: update deps and scripts for Electron-only architecture

- Add electron, concurrently, wait-on, cross-env to frontend devDependencies
- Set "main": "electron/main.cjs" in frontend/package.json
- Simplify root package.json dev script
- Remove fastapi and uvicorn from requirements.txt

package.json

@@ -2,7 +2,7 @@
   "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\""
+    "dev": "cd frontend && npm run dev"
   },
   "devDependencies": {
     "concurrently": "^9.2.1"