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

frontend/package.json

@@ -3,8 +3,10 @@
   "private": true,
   "version": "1.0.0",
   "type": "module",
+  "main": "electron/main.cjs",
   "scripts": {
     "vite":    "vite",
+    "dev":     "concurrently --kill-others -n Vite,Electron \"vite\" \"wait-on http://localhost:5173 && cross-env NODE_ENV=development electron .\"",
     "build":   "vite build",
     "preview": "vite preview"
   },
@@ -17,10 +19,14 @@
     "@types/react": "^19.2.7",
     "@types/react-dom": "^19.2.3",
     "@vitejs/plugin-react": "^5.1.1",
+    "concurrently": "^9.2.1",
+    "cross-env": "^10.1.0",
+    "electron": "^40.8.0",
     "eslint": "^9.39.1",
     "eslint-plugin-react-hooks": "^7.0.1",
     "eslint-plugin-react-refresh": "^0.4.24",
     "globals": "^16.5.0",
-    "vite": "^7.3.1"
+    "vite": "^7.3.1",
+    "wait-on": "^9.0.4"
   }
 }