Add user management page and technician picker for reassign

- New /admin/users page (admin only) for creating technicians,
  editing role/email, resetting passwords, deleting users
- AftersalesDetail reassign modal now uses a searchable Select
  populated from /api/users/assignable instead of raw user ID input
- Menu entry only shown to admins

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/types/index.ts

@@ -1,12 +1,40 @@
+export type UserRole = 'admin' | 'technician' | 'user';
+
 export interface User {
   id: number;
   username: string;
   name: string;
   email?: string;
-  role: string;
+  role: UserRole;
   createdAt: string;
 }
 
+export interface CreateUserRequest {
+  username: string;
+  password: string;
+  name: string;
+  email?: string;
+  role: UserRole;
+}
+
+export interface UpdateUserRequest {
+  name?: string;
+  email?: string;
+  role?: UserRole;
+}
+
+export interface UserListFilter {
+  page?: number;
+  limit?: number;
+  role?: UserRole;
+  search?: string;
+}
+
+export interface UserListResponse {
+  data: User[];
+  pagination: EmployeeSerialPagination;
+}
+
 export interface Company {
   id: number;
   name: string;