fix(SaveManager): use getActiveOrNull to avoid race when leaving editor

When closeProject() sets this.active = null, a debounced save scheduled before
  may still fire, causing getActive() to throw "No active project". The existing
  `if (!activeProject) return;` guard was dead code. Switching to getActiveOrNull
  makes the guard actually work.
    - 选 ⊙ Create a new branch for

apps/web/src/core/managers/save-manager.ts

@@ -80,7 +80,7 @@ export class SaveManager {
 		if (this.isSaving) return;
 		if (!this.hasPendingSave) return;
 
-		const activeProject = this.editor.project.getActive();
+		const activeProject = this.editor.project.getActiveOrNull();
 		if (!activeProject) return;
 		if (this.editor.project.getIsLoading()) return;
 		if (this.editor.project.getMigrationState().isMigrating) return;