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/service/admin_service_create_user_test.go

@@ -18,7 +18,6 @@ func TestAdminService_CreateUser_Success(t *testing.T) {
 		Email:         "user@test.com",
 		Password:      "strong-pass",
 		Username:      "tester",
-		Wechat:        "wx",
 		Notes:         "note",
 		Balance:       12.5,
 		Concurrency:   7,
@@ -31,7 +30,6 @@ func TestAdminService_CreateUser_Success(t *testing.T) {
 	require.Equal(t, int64(10), user.ID)
 	require.Equal(t, input.Email, user.Email)
 	require.Equal(t, input.Username, user.Username)
-	require.Equal(t, input.Wechat, user.Wechat)
 	require.Equal(t, input.Notes, user.Notes)
 	require.Equal(t, input.Balance, user.Balance)
 	require.Equal(t, input.Concurrency, user.Concurrency)