feat: integrate Stripe payment with subscription plans

Add Stripe payment integration across the project:

- Add @stripe/stripe-js dependency
- Configure Stripe environment variables (secret key, publishable key, webhook secret) for dev, local, and production environments
- Add Product/Price IDs for monthly, quarterly, and annual subscription tiers
- Extend subscription plan data with voiceMinutesPerDay quotas (30/45/60 minutes per tier)
- Update .gitignore to exclude implementation_plan.md

package.json

@@ -17,6 +17,7 @@
   },
   "dependencies": {
     "@fingerprintjs/fingerprintjs": "^5.2.0",
+    "@stripe/stripe-js": "^9.8.0",
     "@xstate/react": "^5",
     "classnames": "^2.5.1",
     "dexie": "^4.4.3",
@@ -27,6 +28,7 @@
     "pino": "^10.3.1",
     "react": "19.2.4",
     "react-dom": "19.2.4",
+    "stripe": "^22.2.1",
     "unstorage": "^1.17.5",
     "xstate": "^5",
     "zod": "^4.4.3"