feat: align iOS client safety and consent experience

health_app/lib/pages/settings/ai_consent_details_page.dart

@@ -32,7 +32,15 @@ class AiConsentDetailsPage extends ConsumerWidget {
     );
     if (confirmed != true || !context.mounted) return;
 
-    await ref.read(aiConsentProvider.notifier).revoke(userId);
+    final revoked = await ref.read(aiConsentProvider.notifier).revoke(userId);
+    if (!revoked) {
+      if (context.mounted) {
+        ScaffoldMessenger.of(context).showSnackBar(
+          const SnackBar(content: Text('Authorization could not be revoked.')),
+        );
+      }
+      return;
+    }
     await ref.read(authProvider.notifier).logout();
   }
 
@@ -59,6 +67,7 @@ class AiConsentDetailsPage extends ConsumerWidget {
             color: AppColors.textPrimary,
           ),
         ),
+        centerTitle: true,
       ),
       body: SafeArea(
         child: SingleChildScrollView(
@@ -91,8 +100,11 @@ class AiConsentDetailsPage extends ConsumerWidget {
                 SizedBox(
                   width: double.infinity,
                   child: OutlinedButton(
-                    onPressed: () => _revoke(context, ref),
+                    onPressed: consent.isSaving
+                        ? null
+                        : () => _revoke(context, ref),
                     style: OutlinedButton.styleFrom(
+                      backgroundColor: Colors.white,
                       foregroundColor: AppColors.error,
                       side: BorderSide(
                         color: AppColors.error.withValues(alpha: 0.4),