feat(admin): add dedicated routes for admin user management (#37)

- New GET /admin/users/new page: create user with all profile fields
  (login, password, firstName, lastName, birthDate, email, contact, groups)
- New GET /admin/users/[id] page: edit user profile, groups, and
  optional password change without requiring current password
- New PUT /api/users/{id} backend endpoint (ADMIN_USER permission)
  with AdminUpdateUserRequest DTO for admin-override user updates
- Refactored admin users tab: replaced inline editing with edit links
  to dedicated routes; create button now links to /admin/users/new
- Extended CreateUserRequest with profile fields so new users can be
  created with full profile data in a single request
- Added 28 component tests across 3 new spec files (TDD)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/admin/users/new/+page.server.ts

@@ -0,0 +1,45 @@
+import { error, fail, redirect } from '@sveltejs/kit';
+import type { PageServerLoad, Actions } from './$types';
+import { createApiClient } from '$lib/api.server';
+import { getErrorMessage } from '$lib/errors';
+
+export const load: PageServerLoad = async ({ fetch, locals }) => {
+	const user = locals.user;
+	const hasAdmin = user?.groups?.some((g: { permissions: string[] }) =>
+		g.permissions.includes('ADMIN')
+	);
+	if (!hasAdmin) throw error(403, getErrorMessage('FORBIDDEN'));
+
+	const api = createApiClient(fetch);
+	const groupsResult = await api.GET('/api/groups');
+
+	return { groups: groupsResult.data ?? [] };
+};
+
+export const actions: Actions = {
+	default: async ({ request, fetch }) => {
+		const data = await request.formData();
+		const api = createApiClient(fetch);
+
+		const birthDateRaw = data.get('birthDate') as string;
+		const result = await api.POST('/api/users', {
+			body: {
+				username: data.get('username') as string,
+				initialPassword: data.get('password') as string,
+				email: (data.get('email') as string) || undefined,
+				groupIds: data.getAll('groupIds') as string[],
+				firstName: (data.get('firstName') as string) || null,
+				lastName: (data.get('lastName') as string) || null,
+				birthDate: birthDateRaw || null,
+				contact: (data.get('contact') as string) || null
+			}
+		});
+
+		if (!result.response.ok) {
+			const code = (result.error as unknown as { code?: string })?.code;
+			return fail(result.response.status, { error: getErrorMessage(code) });
+		}
+
+		throw redirect(303, '/admin');
+	}
+};