Add authentication validation on login screen load

- Add validateToken() method to userService to check if refresh token is valid
- Automatically redirect to chat if user already has valid session
- Show 'Checking authentication...' loading state during validation
- Prevents unnecessary login if user is already authenticated
- Improves UX by skipping login screen when not needed

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

raggr-frontend/src/api/userService.ts

@@ -55,6 +55,21 @@ class UserService {
     return data.access_token;
   }
 
+  async validateToken(): Promise<boolean> {
+    const refreshToken = localStorage.getItem("refresh_token");
+
+    if (!refreshToken) {
+      return false;
+    }
+
+    try {
+      await this.refreshToken();
+      return true;
+    } catch (error) {
+      return false;
+    }
+  }
+
   async fetchWithAuth(
     url: string,
     options: RequestInit = {},