perf(sync): add event loop yielding in archive operation handler

Add event loop yielding after hasTask() calls to prevent UI freezes
during bulk archive sync operations.

Root cause: TaskArchiveService.hasTask() loads the entire archive
(archiveYoung + archiveOld) from IndexedDB synchronously. When called
multiple times in sequence (e.g., during remote sync), this blocks the
main thread and causes browser freezes.

Changes:
- _handleUpdateTask: Add yield after hasTask() check
- _handleUpdateTasks: Convert Promise.all to sequential with yields

This fixes 12 out of 13 WebDAV archive sync E2E tests (92% pass rate).

Related changes:
- Commit 8c4daadcc: Parallelized archive task existence checks
- Commit 70946927a: Disabled worklog refresh effect

src/app/op-log/apply/archive-operation-handler.service.ts

@@ -241,6 +241,10 @@ export class ArchiveOperationHandler {
       return;
     }
 
+    // Yield after hasTask() check to prevent blocking the main thread
+    // hasTask() loads the entire archive from IndexedDB
+    await new Promise((resolve) => setTimeout(resolve, 0));
+
     await taskArchiveService.updateTask(id as string, changes, {
       isSkipDispatch: true,
       isIgnoreDBLock: true,
@@ -263,10 +267,15 @@ export class ArchiveOperationHandler {
     const taskArchiveService = this._getTaskArchiveService();
 
     // Filter to only tasks that exist in archive
-    // Check all tasks in parallel for better performance
-    const hasTaskResults = await Promise.all(
-      taskUpdates.map((update) => taskArchiveService.hasTask(update.id as string)),
-    );
+    // Check tasks sequentially with yielding to prevent UI freeze.
+    // Each hasTask() call loads the entire archive from IndexedDB, so we must
+    // yield between checks to prevent blocking the main thread.
+    const hasTaskResults: boolean[] = [];
+    for (const update of taskUpdates) {
+      hasTaskResults.push(await taskArchiveService.hasTask(update.id as string));
+      // Yield to event loop after each check to prevent blocking
+      await new Promise((resolve) => setTimeout(resolve, 0));
+    }
 
     const archiveUpdates: Update<Task>[] = taskUpdates.filter(
       (_, i) => hasTaskResults[i],