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/handler/dto/mappers.go

@@ -10,7 +10,6 @@ func UserFromServiceShallow(u *service.User) *User {
 		ID:            u.ID,
 		Email:         u.Email,
 		Username:      u.Username,
-		Wechat:        u.Wechat,
 		Notes:         u.Notes,
 		Role:          u.Role,
 		Balance:       u.Balance,

backend/internal/handler/dto/types.go

@@ -6,7 +6,6 @@ type User struct {
 	ID            int64     `json:"id"`
 	Email         string    `json:"email"`
 	Username      string    `json:"username"`
-	Wechat        string    `json:"wechat"`
 	Notes         string    `json:"notes"`
 	Role          string    `json:"role"`
 	Balance       float64   `json:"balance"`