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/admin/usage_handler.go

@@ -246,7 +246,7 @@ func (h *UsageHandler) SearchUsers(c *gin.Context) {
 	}
 
 	// Limit to 30 results
-	users, _, err := h.adminService.ListUsers(c.Request.Context(), 1, 30, "", "", keyword)
+	users, _, err := h.adminService.ListUsers(c.Request.Context(), 1, 30, service.UserListFilters{Search: keyword})
 	if err != nil {
 		response.ErrorFrom(c, err)
 		return