feat: add hidden tag to hide hosts from TUI and search

Hosts tagged with "hidden" are excluded from the TUI list, shell
completions, and sshm search. Direct connections via sshm <host>
still work regardless of the tag.

A toggle key (H) shows or hides hidden hosts in the TUI, with a
yellow banner indicating the active state. The key is documented
in the help panel (h).

A contextual hint on the Tags field in the add and edit forms
reminds the user that "hidden" hides the host from the list.

internal/ui/view.go

@@ -86,6 +86,14 @@ func (m Model) renderListView() string {
 		components = append(components, errorStyle.Render("❌ "+m.errorMessage))
 	}
 
+	// Add indicator when hidden hosts are shown
+	if m.showHidden {
+		hiddenBannerStyle := lipgloss.NewStyle().
+			Foreground(lipgloss.Color("11")).
+			Bold(true)
+		components = append(components, hiddenBannerStyle.Render("  [showing hidden hosts — press H to hide]"))
+	}
+
 	// Add the search bar with the appropriate style based on focus
 	searchPrompt := "Search (/ to focus): "
 	if m.searchMode {