fix: implement user management UI

Complete user management frontend with resend invite, email validation,
and modal accessibility improvements.

Backend:

Add POST /api/v1/users/:id/resend-invite endpoint with authorization
Add 6 unit tests for resend invite handler
Fix feature flags default values
Frontend:

Add client-side email format validation with error display
Add resend invite button for pending users with Mail icon
Add Escape key keyboard navigation for modals
Fix PermissionsModal useState anti-pattern (now useEffect)
Add translations for de/es/fr/zh locales
Tests:

Enable 7 previously-skipped E2E tests (now 15 passing)
Fix Playwright locator strict mode violations
Update UsersPage test mocks for new API
Docs:

Document resend-invite API endpoint
Update CHANGELOG for Phase 6

frontend/src/api/users.ts

@@ -201,3 +201,13 @@ export const previewInviteURL = async (email: string): Promise<PreviewInviteURLR
   const response = await client.post<PreviewInviteURLResponse>('/users/preview-invite-url', { email })
   return response.data
 }
+
+/**
+ * Resends an invitation email to a pending user.
+ * @param id - The user ID to resend invite to
+ * @returns Promise resolving to InviteUserResponse with new token
+ */
+export const resendInvite = async (id: number): Promise<InviteUserResponse> => {
+  const response = await client.post<InviteUserResponse>(`/users/${id}/resend-invite`)
+  return response.data
+}