feat(tui): add 13 preloaded themes matching web design system

Port all web CSS token themes to TUI via shared vocabulary (accent, dim,
muted, ok, todo, event, remind, danger). Styles rebuild from active
theme on switch. Press T to cycle, persists to ~/.nib/theme. Glamour
markdown renderer respects light/dark per theme.

internal/tui/help.go

@@ -33,6 +33,7 @@ func renderHelp(width, height int) string {
 			{"2", "cards view"},
 			{"s", "cycle sort (cards)"},
 			{"i", "cycle intent (cards)"},
+			{"T", "cycle theme"},
 		}},
 		{"Actions", [][2]string{
 			{"d", "delete (with confirm)"},