chore: remove orphaned and unused files

- Delete src/api.py (FastAPI backend replaced by Electron IPC)
- Delete frontend/src/components/DecryptWallet.jsx (replaced by WalletViewer)
- Delete frontend/public/vite.svg and src/assets/react.svg (Vite/React scaffolding defaults)
- Simplify root package.json (remove unused concurrently devDependency)

package.json

@@ -1,10 +1,7 @@
 {
-  "name": "bitcoin-address-generator",
+  "name": "address-gen",
   "private": true,
   "scripts": {
     "dev": "cd frontend && npm run dev"
-  },
-  "devDependencies": {
-    "concurrently": "^9.2.1"
   }
 }