Add main menu page with role-based modules (#7)

- `/` is now the main menu; orders/receipts page moved to `/receipts`
- Module registry (features/menu/modules.ts) drives both the menu tiles
  and route guards (ModuleRoute); admin sees every module
- Menu: module search with Ctrl+K, profile with logout, responsive grid,
  palette matching the login page
- «← Головне меню» link on receipts and cash registers pages;
  «Каси» moved from the receipts toolbar into the menu

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/features/auth/roles.ts

@@ -0,0 +1,7 @@
+import type { UserRole } from '@/api/types'
+
+export const ROLE_LABEL: Record<UserRole, string> = {
+  admin: 'Адміністратор',
+  cashier: 'Касир',
+  viewer: 'Спостерігач',
+}