feat(feedback): implement user feedback collection system with email reporting

Backend:
- Add user_feedback table with RLS policy
- Create feedback submission API (multipart/form-data)
- Implement xlsx report generation with embedded images
- Add scheduled email delivery via Feishu SMTP
- Create HTML email templates (daily_report, no_feedback)

Frontend:
- Add feedback screen with type selection and image picker
- Support anonymous submission via skipAuth flag
- Collect device info and app version

Protocol:
- Document feedback API contract and error codes
- Update http-error-codes.md with FEEDBACK_* codes

apps/lib/l10n/app_localizations_en.dart

@@ -1224,4 +1224,62 @@ class AppLocalizationsEn extends AppLocalizations {
 
   @override
   String get settingsDoNotSellDisabled => 'On';
+
+  @override
+  String get settingsFeedbackTitle => 'Feedback';
+
+  @override
+  String get feedbackTitle => 'Feedback';
+
+  @override
+  String get feedbackTypeLabel => 'Feedback Type';
+
+  @override
+  String get feedbackTypeBug => 'Bug';
+
+  @override
+  String get feedbackTypeSuggestion => 'Suggestion';
+
+  @override
+  String get feedbackTypeOther => 'Other';
+
+  @override
+  String get feedbackContentLabel => 'Content';
+
+  @override
+  String get feedbackContentHint =>
+      'Please describe your issue or suggestion in detail...';
+
+  @override
+  String get feedbackImagesLabel => 'Add Screenshots (max 3)';
+
+  @override
+  String get feedbackAnonymousLabel => 'Do not upload my personal information';
+
+  @override
+  String get feedbackAnonymousHint =>
+      'If checked, your user ID will not be collected. Device info will still be collected for troubleshooting.';
+
+  @override
+  String get feedbackSubmit => 'Submit Feedback';
+
+  @override
+  String get feedbackSubmitting => 'Submitting...';
+
+  @override
+  String get feedbackSuccess =>
+      'Thank you for your feedback. We will process it soon.';
+
+  @override
+  String get feedbackContentRequired => 'Please enter feedback content';
+
+  @override
+  String get feedbackContentTooLong =>
+      'Feedback content cannot exceed 500 characters';
+
+  @override
+  String get feedbackTooManyImages => 'Maximum 3 images allowed';
+
+  @override
+  String get feedbackImageTooLarge => 'Image size cannot exceed 5MB';
 }