feat(i18n): localize onboarding & reports; logout

- add en/ms keys for welcome, onboarding, capture/review, errors, and
  settings labels
- replace hardcoded strings with I18n.t in WelcomeScreen, OnboardingFlow,
  CaptureScreen, ReviewScreen, Map severity legend, and Settings
- update StartRouter: onboarded guests go to MainScreen; non-guest users
  see SignIn; skip marks user as onboarded guest
- add logout action in Settings that clears onboarding/user mode prefs
  and returns to the welcome flow
- refine settings diagnostics and app section; move labels to i18n
- leave TODOs for app subtitle, processing status, and ReportCard menu
  labels for future i18n coverage

lib/widgets/report_card.dart

@@ -380,7 +380,9 @@ class ReportCard extends StatelessWidget {
                         children: [
                           Icon(Icons.visibility),
                           SizedBox(width: 8),
-                          Text('View Details'),
+                          Text(
+                            'View Details',
+                          ), // TODO: Move to i18n but need to handle dynamic text in popup menu
                         ],
                       ),
                     ),
@@ -390,7 +392,9 @@ class ReportCard extends StatelessWidget {
                         children: [
                           Icon(Icons.update),
                           SizedBox(width: 8),
-                          Text('Update Status'),
+                          Text(
+                            'Update Status',
+                          ), // TODO: Move to i18n but need to handle dynamic text in popup menu
                         ],
                       ),
                     ),
@@ -400,7 +404,10 @@ class ReportCard extends StatelessWidget {
                         children: [
                           Icon(Icons.delete, color: Colors.red),
                           SizedBox(width: 8),
-                          Text('Delete', style: TextStyle(color: Colors.red)),
+                          Text(
+                            'Delete',
+                            style: TextStyle(color: Colors.red),
+                          ), // TODO: Move to i18n but need to handle dynamic text in popup menu
                         ],
                       ),
                     ),