refactor(sftp): decouple transfers from SFTP panel lifecycle

[CoolLoong]

Problem

Closing the SFTP page during an upload/download freezes the progress bar at InProgress and breaks the cancel button.

Root cause:

transfer task lifecycle was owned by SftpTable, and progress/completion/cancel were reported back through a WeakEntity<TableState>. Once the page closed and the entity was dropped, the reporting closures stopped running, so the global TransferCenterState stayed stuck at InProgress and the cancel token became unreachable.

Solution

Move transfer task ownership from SftpTable to the global TransferCenterState:

• Removed Transfer / TransferEntry / SftpTable.transfers and the table/transfers.rs module.
• Upload/download now write to the global center via AsyncApp; the copy loop no longer depends on a WeakEntity, so it survives panel close.
• Failure reasons are written to TransferTask.detail, so the footbar tooltip still shows them after the panel is gone.
• disconnect() no longer touches transfer tasks, so closing the page never cancels in-flight transfers.
• Terminal-state tasks auto-dismiss after AUTO_DISMISS_AFTER.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 298 lines in your changes missing coverage. Please review.
✅ Project coverage is 54.56%. Comparing base (161e688) to head (8f59f96).

Files with missing lines	Patch %	Lines
crates/gpui_sftp/src/view/table/actions.rs	0.00%	298 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #43      +/-   ##
==========================================
+ Coverage   54.53%   54.56%   +0.02%     
==========================================
  Files         159      158       -1     
  Lines       54170    53906     -264     
==========================================
- Hits        29542    29413     -129     
+ Misses      24628    24493     -135     

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