feat(app): add error/loading boundaries and update dashboard routes

- Add error.tsx and loading.tsx boundaries for admin, parent, student, teacher routes

- Add dashboard-error-fallback and dashboard-loading-skeleton components

- Add student/learning page, parent/leave routes, teacher textbook components

- Update existing app routes across auth, dashboard, and API endpoints

- Update proxy middleware and next-auth type declarations

src/next-auth.d.ts

@@ -8,6 +8,7 @@ declare module "next-auth" {
       role: string // kept for backward compatibility
       roles: Role[]
       permissions: Permission[]
+      onboarded: boolean
     }
   }
 }
@@ -18,5 +19,6 @@ declare module "next-auth/jwt" {
     role: string // kept for backward compatibility
     roles: Role[]
     permissions: Permission[]
+    onboarded: boolean
   }
 }