feat: implement admin dashboard with site settings, user management, and activity monitoring

frontend/src/types.ts

@@ -113,7 +113,7 @@ export interface GradeResult {
   points: number
 }
 
-export type AppScreen = 'landing' | 'exam' | 'results' | 'review' | 'verify' | 'reset-password'
+export type AppScreen = 'landing' | 'exam' | 'results' | 'review' | 'verify' | 'reset-password' | 'admin'
 export type ThemeMode = 'light' | 'dark'
 export type FontSize = 'small' | 'medium' | 'large'
 export type FeedbackTweak = 'auto' | 'immediate' | 'end'