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/l10n/app_localizations_en.dart

@@ -1211,4 +1211,17 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get settingsInviteInvalidCode =>
       'Please enter a valid 6-character invite code';
+
+  @override
+  String get settingsDoNotSellTitle => 'Personalized Ads';
+
+  @override
+  String get settingsDoNotSellDescription =>
+      'When off, your personal info won\'t be used for ad recommendations';
+
+  @override
+  String get settingsDoNotSellEnabled => 'Off';
+
+  @override
+  String get settingsDoNotSellDisabled => 'On';
 }