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/validators/adminSchemas.ts

@@ -0,0 +1,20 @@
+import { z } from 'zod'
+
+export const updateUserRoleSchema = z.object({
+  user_id: z.number().min(1),
+  role: z.enum(['user', 'admin']),
+})
+
+export const updateSettingSchema = z.object({
+  key: z.string().min(1),
+  value: z.string().min(1),
+})
+
+export const listUsersQuerySchema = z.object({
+  page: z.string().regex(/^\d+$/).optional().transform(v => (v ? Number(v) : 1)),
+  search: z.string().optional(),
+})
+
+export const userIdParamSchema = z.object({
+  id: z.string().regex(/^\d+$/, 'ID must be a number'),
+})