feat: Implement API modules for admin, auth, exam, result, and setup functionalities

- Created adminApi.ts for admin-related operations including user management and site settings.
- Developed authApi.ts for authentication processes such as registration, login, and profile management.
- Added examApi.ts for handling exam CRUD operations.
- Introduced resultApi.ts for managing results submission and retrieval.
- Established setupApi.ts for initial setup processes and status checks.
- Created ErrorBoundary component to handle rendering errors gracefully in the frontend.
- Re-exported shared API utilities in index.ts for easier access.

backend/src/repositories/examRepo.ts

@@ -0,0 +1,67 @@
+import { getDb } from '../db/database.js'
+
+export interface DbExam {
+  id: number
+  user_id: number
+  title: string
+  subtitle: string | null
+  exam_json: string
+  created_at: number
+  updated_at: number
+}
+
+export interface ExamListItem {
+  id: number
+  title: string
+  subtitle: string | null
+  created_at: number
+  updated_at: number
+  question_count: number
+}
+
+export function createExam(userId: number, title: string, subtitle: string | null, examJson: string): { id: number } {
+  const db = getDb()
+  const result = db.prepare(`
+    INSERT INTO saved_exams (user_id, title, subtitle, exam_json)
+    VALUES (?, ?, ?, ?)
+  `).run(userId, title, subtitle, examJson)
+  return { id: result.lastInsertRowid as number }
+}
+
+export function getExamById(id: number, userId: number): DbExam | undefined {
+  const db = getDb()
+  return db.prepare('SELECT * FROM saved_exams WHERE id = ? AND user_id = ?').get(id, userId) as DbExam | undefined
+}
+
+export function listExamsByUser(userId: number): ExamListItem[] {
+  const db = getDb()
+  return db.prepare(`
+    SELECT id, title, subtitle, created_at, updated_at,
+           json_array_length(json_extract(exam_json, '$.questions')) AS question_count
+    FROM saved_exams
+    WHERE user_id = ?
+    ORDER BY updated_at DESC
+  `).all(userId) as ExamListItem[]
+}
+
+export function updateExam(id: number, userId: number, title: string, subtitle: string | null, examJson: string): boolean {
+  const db = getDb()
+  const result = db.prepare(`
+    UPDATE saved_exams
+    SET title = ?, subtitle = ?, exam_json = ?, updated_at = unixepoch()
+    WHERE id = ? AND user_id = ?
+  `).run(title, subtitle, examJson, id, userId)
+  return result.changes > 0
+}
+
+export function deleteExam(id: number, userId: number): boolean {
+  const db = getDb()
+  const result = db.prepare('DELETE FROM saved_exams WHERE id = ? AND user_id = ?').run(id, userId)
+  return result.changes > 0
+}
+
+export function countExams(): number {
+  const db = getDb()
+  const row = db.prepare('SELECT COUNT(*) AS cnt FROM saved_exams').get() as { cnt: number }
+  return row.cnt
+}