fix: critical navigation bug - users trapped in detail and restore screens

Changed return value from 'return s, nil' to 'return nil, nil' in:
- detail.go (line 116): Back navigation from client details
- restore.go (line 101): Back navigation from restore screen

Root cause: Main model's navigation logic checks if newScreen == nil.
When screens returned (s, nil), the check failed and screen never changed.
Now properly returns (nil, nil) to signal screen change to parent.

Fixes: wg-admin-rfo

internal/tui/screens/restore.go

@@ -98,7 +98,7 @@ func (s *RestoreScreen) Update(msg tea.Msg) (Screen, tea.Cmd) {
 		switch msg.String() {
 		case "q", "esc":
 			// Return to list screen - signal parent to switch screens
-			return s, nil
+			return nil, nil
 		case "enter":
 			// Show confirmation for selected backup
 			if len(s.table.Rows()) > 0 {