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/model.go

@@ -105,6 +105,8 @@ type model struct {
 }
 
 func newModel(store *db.Store) model {
+	loadTheme()
+	applyTheme()
 	inp := newInputModel()
 	inp.ti.Focus()
 	return model{
@@ -565,6 +567,10 @@ func (m model) updateBrowse(msg tea.KeyMsg) (tea.Model, tea.Cmd) {
 		}
 		return m, nil
 
+	case "T":
+		t := cycleTheme()
+		return m, m.setStatus("theme: " + t.Name)
+
 	case "i":
 		if m.mode == modeCards && m.state == stateList {
 			m.cards.setIntent(m.cards.intent.next())