[codex] Support account deletion feedback

[jjoonleo]

Summary

• send optional withdrawal feedback in account deletion DELETE bodies
• route normal, Google, and Apple account deletion through the correct backend endpoints
• keep users signed in on deletion failure, and explicitly redirect to sign-in after successful deletion
• add focused tests for deletion payloads and social account routing

Why

The backend now accepts optional feedback directly on account deletion requests. The previous frontend flow posted feedback separately and could sign users out even when deletion failed, which made social revoke errors look like successful withdrawal.

Validation

• flutter analyze
• flutter test test/data/data_sources/authentication_remote_data_source_test.dart test/domain/use-cases/delete_user_use_case_test.dart test/presentation/my_page/delete_user_modal_test.dart

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