V3-12: crash reporting (code only)

shouldInitializeCrashReporting() and scrubExtra() are pure, fully unit-tested decision/scrubbing functions wired into sentry_flutter via beforeSend/beforeBreadcrumb. Config.crashReportingEnabled defaults off; the DSN is a compile-time --dart-define, not a preference. RecordingEngine gained an injected onUnexpectedStop callback, firing once when restoreAfterProcessDeath finds a trip still 'recording' at launch -- the process died without a user stop.

Marked partially done: the ticket's real acceptance criteria (a forced crash landing in a real Sentry project from a release build, inspecting a real payload) need a Sentry account and a release build this environment can't produce.

lib/src/ui/settings/settings_screen.dart

@@ -142,6 +142,21 @@ class _SettingsBodyState extends ConsumerState<_SettingsBody> {
           },
         ),
         const Divider(),
+        const _SectionHeader('Crash reporting'),
+        SwitchListTile(
+          key: const Key('crash-reporting-switch'),
+          title: const Text('Send crash reports'),
+          subtitle: const Text(
+            'Off by default. No location, device id, or ride content is ever included '
+            '-- see V3-12. Takes effect on next launch.',
+          ),
+          value: ref.watch(crashReportingEnabledProvider),
+          onChanged: (value) async {
+            await widget.config.setCrashReportingEnabled(value);
+            ref.read(crashReportingEnabledProvider.notifier).state = value;
+          },
+        ),
+        const Divider(),
         const _SectionHeader('Sync'),
         Padding(
           padding: const EdgeInsets.symmetric(horizontal: 16),