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/tui.go

@@ -76,6 +76,11 @@ func (m *Model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 		cmd := m.tabs.Add(tab)
 		return m, cmd
 
+	case openSFTPMsg:
+		tab := NewSFTPBrowserTab(msg.host, m.dataDir)
+		cmd := m.tabs.Add(tab)
+		return m, cmd
+
 	case closeFormMsg:
 		if m.tabs.Len() > 1 {
 			m.tabs.CloseActive()