feat: integrate Stripe payment with bugfixes and active timeout cancellation

- Add Stripe payment provider with Checkout Session flow
- Payment provider abstraction layer (EasyPay + Stripe unified interface)
- Stripe webhook with proper raw body handling and signature verification
- Frontend: Stripe button with URL validation, anti-duplicate click, noopener
- Active timeout cancellation: query platform before expiring, recover paid orders
- Singleton Stripe client, idempotency keys, Math.round for amounts
- Handle async_payment events, return null for unknown webhook events
- Set Checkout Session expires_at aligned with order timeout
- Add cancelPayment to provider interface (Stripe: sessions.expire, EasyPay: no-op)
- Enable stripe in frontend payment type list

eslint.config.mjs

@@ -1,6 +1,6 @@
-import { defineConfig, globalIgnores } from "eslint/config";
-import nextVitals from "eslint-config-next/core-web-vitals";
-import nextTs from "eslint-config-next/typescript";
+import { defineConfig, globalIgnores } from 'eslint/config';
+import nextVitals from 'eslint-config-next/core-web-vitals';
+import nextTs from 'eslint-config-next/typescript';
 
 const eslintConfig = defineConfig([
   ...nextVitals,
@@ -8,10 +8,12 @@ const eslintConfig = defineConfig([
   // Override default ignores of eslint-config-next.
   globalIgnores([
     // Default ignores of eslint-config-next:
-    ".next/**",
-    "out/**",
-    "build/**",
-    "next-env.d.ts",
+    '.next/**',
+    'out/**',
+    'build/**',
+    'next-env.d.ts',
+    // Git submodules:
+    'vendor/**',
   ]),
 ]);