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/screens/map/map_screen.dart

@@ -679,7 +679,7 @@ class _MapScreenState extends State<MapScreen> {
                             ),
                             const SizedBox(width: 8),
                             Text(
-                              'Issue Severity',
+                              I18n.t('label.severity'),
                               style: Theme.of(context).textTheme.bodySmall
                                   ?.copyWith(
                                     fontWeight: FontWeight.w600,
@@ -691,11 +691,20 @@ class _MapScreenState extends State<MapScreen> {
                           ],
                         ),
                         const SizedBox(height: 12),
-                        _enhancedLegendItem(Severity.high, 'High Priority'),
+                        _enhancedLegendItem(
+                          Severity.high,
+                          '${I18n.t('severity.high')} Priority',
+                        ),
                         const SizedBox(height: 8),
-                        _enhancedLegendItem(Severity.medium, 'Medium Priority'),
+                        _enhancedLegendItem(
+                          Severity.medium,
+                          '${I18n.t('severity.medium')} Priority',
+                        ),
                         const SizedBox(height: 8),
-                        _enhancedLegendItem(Severity.low, 'Low Priority'),
+                        _enhancedLegendItem(
+                          Severity.low,
+                          '${I18n.t('severity.low')} Priority',
+                        ),
                       ],
                     ),
                   ),