phase3: nullfix.py batch script — bang/late/param-nullable

Automated null-safety fixes driven by dart analyze (no corruption):
- unchecked_use_of_nullable_value: insert '!' on receiver (property/method/[]/op)
- not_initialized field/var: mark 'late'
- missing_default_value_for_parameter: nullable param
Errors: 2374(peak) -> 907

lib/widgets/desktop/desktop_new_ticket.dart

@@ -155,7 +155,7 @@ class DesktopNewTicketState extends State<DesktopNewTicket> {
                               ),
                               autofocus: true,
                               validator: (String? value) {
-                                if (value.trim().isEmpty) {
+                                if (value!.trim().isEmpty) {
                                   return S.of(context).this_field_is_required;
                                 }
                                 return null;