feat: TUI SFTP Browser Tab - Task 18

- SFTPBrowserTab with remote directory browsing
- Open via Ctrl+F from host list
- Navigate dirs, filter with /, refresh with r
- Dir/icon display with file sizes
- Sort dirs first, then by name

pkg/tui/messages.go

@@ -45,6 +45,11 @@ type saveHostResultMsg struct {
 	err  error
 }
 
+// openSFTPMsg signals the TUI to open an SFTP browser tab
+type openSFTPMsg struct {
+	host *models.Host
+}
+
 // loadedHostsMsg is produced after reloading hosts from storage
 type loadedHostsMsg struct {
 	hosts []*models.Host