feat: Implement staff invitation system with role-based permissions

- Add StaffInvitation model with token-based 7-day expiration
- Create invitation API endpoints (create, cancel, resend, accept, decline)
- Add permissions JSONField to User model for granular access control
- Implement frontend invite modal with role-specific permissions:
  - Manager: can_invite_staff, can_manage_resources, can_manage_services,
    can_view_reports, can_access_settings, can_refund_payments
  - Staff: can_view_all_schedules, can_manage_own_appointments
- Add edit staff modal with permissions management and deactivate option
- Create AcceptInvitePage for invitation acceptance flow
- Add active/inactive staff separation with collapsible section
- Auto-create bookable resource when configured at invite time
- Remove Quick Add Appointment from dashboard

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

frontend/src/App.tsx

@@ -53,6 +53,7 @@ import PlatformSettings from './pages/platform/PlatformSettings';
 import ProfileSettings from './pages/ProfileSettings';
 import VerifyEmail from './pages/VerifyEmail';
 import EmailVerificationRequired from './pages/EmailVerificationRequired';
+import AcceptInvitePage from './pages/AcceptInvitePage';
 
 const queryClient = new QueryClient({
   defaultOptions: {
@@ -210,6 +211,7 @@ const AppContent: React.FC = () => {
         <Route path="/login" element={<LoginPage />} />
         <Route path="/oauth/callback/:provider" element={<OAuthCallback />} />
         <Route path="/verify-email" element={<VerifyEmail />} />
+        <Route path="/accept-invite" element={<AcceptInvitePage />} />
         <Route path="*" element={<Navigate to="/" replace />} />
       </Routes>
     );
@@ -222,6 +224,7 @@ const AppContent: React.FC = () => {
         <Route path="/login" element={<LoginPage />} />
         <Route path="/oauth/callback/:provider" element={<OAuthCallback />} />
         <Route path="/verify-email" element={<VerifyEmail />} />
+        <Route path="/accept-invite" element={<AcceptInvitePage />} />
         <Route path="*" element={<Navigate to="/login" replace />} />
       </Routes>
     );