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_login.dart

@@ -26,9 +26,9 @@ class DesktopLoginState extends State<DesktopLogin> {
 
   final usernameController = TextEditingController();
   final passwordController = TextEditingController();
-  bool passwordVisible;
+  late bool passwordVisible;
 
-  bool onSubmitting;
+  late bool onSubmitting;
 
   double sideSpace = 0;
   double mainSpace = 1200;
@@ -150,7 +150,7 @@ class DesktopLoginState extends State<DesktopLogin> {
                                 style: TextStyle(fontSize: 18.0),
                                 autofocus: true,
                                 validator: (String? value) {
-                                  if (value.trim().isEmpty) {
+                                  if (value!.trim().isEmpty) {
                                     return S.of(context).this_field_is_required;
                                   }
                                   return null;
@@ -194,7 +194,7 @@ class DesktopLoginState extends State<DesktopLogin> {
                                 style: TextStyle(fontSize: 18.0),
                                 obscureText: passwordVisible,
                                 validator: (String? value) {
-                                  if (value.trim().isEmpty) {
+                                  if (value!.trim().isEmpty) {
                                     return S.of(context).password_is_required;
                                   }
                                   return null;