refactor: migrate wechat to user attributes and enhance users list

Migrate the hardcoded wechat field to the new extensible user
attributes system and improve the users management UI.

Migration:
- Add migration 019 to move wechat data to user_attribute_values
- Remove wechat field from User entity, DTOs, and API contracts
- Clean up wechat-related code from backend and frontend

UsersView enhancements:
- Add text labels to action buttons (Filter Settings, Column Settings,
  Attributes Config) for better UX
- Change status column to show colored dot + Chinese text instead of
  English text
- Add dynamic attribute columns support with batch loading
- Add column visibility settings with localStorage persistence
- Add filter settings modal for search and filter preferences
- Update i18n translations

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/internal/repository/fixtures_integration_test.go

@@ -40,7 +40,6 @@ func mustCreateUser(t *testing.T, client *dbent.Client, u *service.User) *servic
 		SetBalance(u.Balance).
 		SetConcurrency(u.Concurrency).
 		SetUsername(u.Username).
-		SetWechat(u.Wechat).
 		SetNotes(u.Notes)
 	if !u.CreatedAt.IsZero() {
 		create.SetCreatedAt(u.CreatedAt)