feat: clear screen on SSH enter/exit, host list scrolling, 'e' edit key

- tea.Batch(tea.ClearScreen, sshConnectCmd) before SSH starts so TUI
  output doesn't bleed into SSH terminal
- tea.ClearScreen after SSH exits so SSH output doesn't ghost in TUI
- Host list now paginates: only (term_height - 8) / 5 cards shown,
  viewport follows selectedIndex with centered scrolling
- Shows 'N of M hosts' indicator when content is clipped
- Added 'e' key alias for editing host (Ctrl+E or E)

pkg/tui/tui.go

@@ -62,13 +62,13 @@ func (m *Model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 		return m, tea.Quit
 
 	case sshConnectToMsg:
-		return m, sshConnectCmd(msg.host, m.dataDir)
+		return m, tea.Batch(tea.ClearScreen, sshConnectCmd(msg.host, m.dataDir))
 
 	case sshExitMsg:
 		if msg.err != nil {
 			m.Error = msg.err
 		}
-		return m, nil
+		return m, tea.ClearScreen
 
 	case openHostFormMsg:
 		var tab Tab