fix(integration): pass .env vars to serve process

Production servers like react-router-serve don't auto-load .env files
(unlike Vite dev or srvx CLI). Read the .env file and pass all vars as
process env to the serve command so Clerk keys are available at runtime.

Author: jacekradko

integration/models/application.ts

@@ -174,9 +174,26 @@ export const application = (
         return { port, serverUrl: runtimeServerUrl };
       }
 
+      // Read .env file and pass as process env vars since production servers
+      // may not auto-load .env files (e.g., react-router-serve)
+      const envFromFile: Record<string, string> = {};
+      const envFilePath = path.resolve(appDirPath, '.env');
+      if (fs.existsSync(envFilePath)) {
+        const envContent = fs.readFileSync(envFilePath, 'utf-8');
+        for (const line of envContent.split('\n')) {
+          const trimmed = line.trim();
+          if (trimmed && !trimmed.startsWith('#')) {
+            const eqIdx = trimmed.indexOf('=');
+            if (eqIdx > 0) {
+              envFromFile[trimmed.slice(0, eqIdx)] = trimmed.slice(eqIdx + 1);
+            }
+          }
+        }
+      }
+
       const proc = run(scripts.serve, {
         cwd: appDirPath,
-        env: { PORT: port.toString() },
+        env: { ...envFromFile, PORT: port.toString() },
         detached: opts.detached,
         stdout: opts.detached ? fs.openSync(stdoutFilePath, 'a') : undefined,
         stderr: opts.detached ? fs.openSync(stderrFilePath, 'a') : undefined,