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/ent/schema/user.go

@@ -57,9 +57,7 @@ func (User) Fields() []ent.Field {
 		field.String("username").
 			MaxLen(100).
 			Default(""),
-		field.String("wechat").
-			MaxLen(100).
-			Default(""),
+		// wechat field migrated to user_attribute_values (see migration 019)
 		field.String("notes").
 			SchemaType(map[string]string{dialect.Postgres: "text"}).
 			Default(""),
@@ -75,6 +73,7 @@ func (User) Edges() []ent.Edge {
 		edge.To("allowed_groups", Group.Type).
 			Through("user_allowed_groups", UserAllowedGroup.Type),
 		edge.To("usage_logs", UsageLog.Type),
+		edge.To("attribute_values", UserAttributeValue.Type),
 	}
 }