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/mobile/mobile_new_ticket.dart

@@ -111,7 +111,7 @@ class MobileNewTicketState extends State<MobileNewTicket> {
                   ),
                   autofocus: true,
                   validator: (String? value) {
-                    if (value.trim().isEmpty) {
+                    if (value!.trim().isEmpty) {
                       return S.of(context).this_field_is_required;
                     }
                     return null;