phase3: bulk null-safety fixes

- validator: (String value) -> (String? value) [30 files]
- ElevatedButton.styleFrom(primary:) -> backgroundColor: [31 files]
- global Key? param fix [129 files]

lib/widgets/mobile/mobile_user_profile.dart

@@ -392,7 +392,7 @@ class MobileUserProfileState extends State<MobileUserProfile> {
                       fontSize: 18.0
                   ),
                   autofocus: true,
-                  validator: (String value) {
+                  validator: (String? value) {
                     if (value.trim().isEmpty) {
                       return S.of(context).nickname_is_required;
                     }