feat: AI cost tracking, pre-launch balance check, low balance alerts

- store/ai_charge.go: local AI cost tracking per call (SQLite)
- wallet/usdc.go: shared USDC balance query (Base chain RPC)
- Pre-launch: estimate daily cost + runway days
- Low balance: warn <$1, error at $0 (every 10 cycles)
- API: GET /api/ai-costs for cost history
- Frontend: model cards show price per call
- Frontend: wallet create + QR deposit + balance display

web/package.json

@@ -24,6 +24,7 @@
     "katex": "^0.16.27",
     "lightweight-charts": "^5.1.0",
     "lucide-react": "^0.552.0",
+    "qrcode.react": "^4.2.0",
     "react": "^18.3.1",
     "react-dom": "^18.3.1",
     "react-password-checklist": "^1.8.1",