feat(frontend): Comprehensive RBAC implementation across admin pages

**Option 3 Implementation (Latest):**
- InviteStaffDialog: Use /admin/roles/assignable endpoint
- AdminStaff: Enable admin users to see 'Invite Staff' button

**Permission Checks Added (8 admin pages):**
- AdminNewsletters: newsletters.create/edit/delete
- AdminFinancials: financials.create/edit/delete
- AdminBylaws: bylaws.create/edit/delete
- AdminValidations: users.approve, subscriptions.activate
- AdminSubscriptions: subscriptions.export/edit/cancel
- AdminDonations: donations.export
- AdminGallery: gallery.upload/edit/delete
- AdminPlans: subscriptions.plans

**Pattern Established:**
All admin action buttons now wrapped with hasPermission() checks.
UI hides what users can't access, backend enforces rules.

**Files Modified:** 10 files, 100+ permission checks added

src/pages/admin/AdminStaff.js

@@ -16,7 +16,7 @@ import { UserCog, Search, Shield, UserPlus, Mail, Edit, Eye, Trash2, UserCheck,
 
 const AdminStaff = () => {
   const navigate = useNavigate();
-  const { hasPermission } = useAuth();
+  const { hasPermission, user } = useAuth();
   const [users, setUsers] = useState([]);
   const [filteredUsers, setFilteredUsers] = useState([]);
   const [loading, setLoading] = useState(true);
@@ -142,7 +142,7 @@ const AdminStaff = () => {
             </p>
           </div>
           <div className="flex gap-3">
-            {hasPermission('users.invite') && (
+            {hasPermission('users.create') && (
               <Button
                 onClick={() => setInviteDialogOpen(true)}
                 className="bg-[#664fa3] hover:bg-[#422268] text-white rounded-xl h-12 px-6"