feat(time-off): Reset approval when staff edits approved request

- Add pre_save signal to track changes to approved time blocks
- Reset to PENDING status when staff modifies approved time-off
- Send re-approval notifications to managers with changed fields
- Update email templates for modified requests
- Allow managers to have self-approval permission revoked (default: allowed)

A changed request is treated as a new request requiring re-approval.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/components/NotificationDropdown.tsx

@@ -57,7 +57,8 @@ const NotificationDropdown: React.FC<NotificationDropdownProps> = ({ variant = '
     }
 
     // Handle time-off request notifications - navigate to time blocks page
-    if (notification.data?.type === 'time_off_request') {
+    // Includes both new requests and modified requests that need re-approval
+    if (notification.data?.type === 'time_off_request' || notification.data?.type === 'time_off_request_modified') {
       navigate('/time-blocks');
       setIsOpen(false);
       return;
@@ -79,8 +80,8 @@ const NotificationDropdown: React.FC<NotificationDropdownProps> = ({ variant = '
   };
 
   const getNotificationIcon = (notification: Notification) => {
-    // Check for time-off request type in data
-    if (notification.data?.type === 'time_off_request') {
+    // Check for time-off request type in data (new or modified)
+    if (notification.data?.type === 'time_off_request' || notification.data?.type === 'time_off_request_modified') {
       return <Clock size={16} className="text-amber-500" />;
     }