fix: Keep completed files when deleting tasks

Modified delete_task() to only delete files for incomplete downloads.
Completed download files are now preserved when cleanup button is used.

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

app/download_manager.py

@@ -76,7 +76,7 @@ class DownloadManager:
                 os.remove(task.file_path)
 
     async def delete_task(self, task_id: str):
-        """Completely remove a task from the task list"""
+        """Completely remove a task from the task list (keeps completed files)"""
         task = self.tasks.get(task_id)
         if task:
             # Cancel if downloading
@@ -84,9 +84,10 @@ class DownloadManager:
                 self.active_downloads[task_id].cancel()
                 del self.active_downloads[task_id]
 
-            # Delete file if exists
-            if task.file_path and os.path.exists(task.file_path):
-                os.remove(task.file_path)
+            # Delete partial file ONLY if download is not completed
+            if task.status != DownloadStatus.COMPLETED:
+                if task.file_path and os.path.exists(task.file_path):
+                    os.remove(task.file_path)
 
             # Remove from tasks dict
             del self.tasks[task_id]