feat(privacy): add personalized ads toggle in settings

- Add PrivacySettings schema with can_sell field (default: false)
- Move privacy toggle to GeneralSettingsScreen with clear labeling
- Update l10n for zh/en/zh_hant with user-friendly copy
- Clean up redundant PrivacyNotificationSettingsScreen
- Update profile-protocol.md to reflect new privacy schema
- Fix basedpyright warnings in user.py

apps/lib/features/settings/presentation/screens/general_settings_screen.dart

@@ -104,6 +104,21 @@ class _GeneralSettingsScreenState extends State<GeneralSettingsScreen> {
               ],
             ),
             const SizedBox(height: AppSpacing.lg),
+            SectionLabel(text: l10n.settingsSectionPrivacy),
+            SettingsGroupCard(
+              children: [
+                SettingsSwitchTile(
+                  icon: Icons.security_rounded,
+                  title: l10n.settingsDoNotSellTitle,
+                  value: _settings.privacy.canSell,
+                  tint: colors.primary,
+                  background: colors.surfaceContainerHighest,
+                  showDivider: false,
+                  onChanged: (value) => _updatePrivacy(canSell: value),
+                ),
+              ],
+            ),
+            const SizedBox(height: AppSpacing.lg),
             SectionLabel(text: l10n.settingsSectionNotification),
             SettingsGroupCard(
               children: [
@@ -151,6 +166,15 @@ class _GeneralSettingsScreenState extends State<GeneralSettingsScreen> {
     await widget.onSettingsChanged(_settings);
   }
 
+  void _updatePrivacy({bool? canSell}) {
+    final newPrivacy = _settings.privacy.copyWith(canSell: canSell);
+    final newSettings = _settings.copyWith(privacy: newPrivacy);
+    setState(() {
+      _settings = newSettings;
+    });
+    widget.onSettingsChanged(newSettings);
+  }
+
   void _updateNotification({bool? allowNotifications, bool? allowVibration}) {
     final newNotification = _settings.notification.copyWith(
       allowNotifications: allowNotifications,