* feat(sftp): add Open with system default for native Windows file association (fixes#1236)
Adds a new 'Open with system default' option to the SFTP context menu
that uses Electron's shell.openPath() to invoke the native OS file opening
mechanism. On Windows, this uses ShellExecute, which works with UWP/WinUI
apps (Photos, Paint, etc.) whose executable paths change with updates.
The existing 'Open with...' (browse for executable) is preserved.
Closes#1236
* fix(sftp): add file watch support to openWithSystemDefault for SFTP auto-sync
P2: The new default-app open flow was not starting a file watch
when SFTP auto-sync was enabled, so edits saved in the system
default app were not uploaded back to the remote host.
Mirrors the downloadToTempAndOpen behavior — accepts an optional
{ enableWatch } parameter and starts a file watch when set.
* fix(sftp): mark transfer as failed when system default open fails
P2: When shell.openPath fails for a remote file, the transfer queue
still shows success because downloadToTemp had already completed the
temp download. Now updates externalTransferId to 'failed' before
throwing, matching downloadToTempAndOpen's behavior.
陈大猫