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/keys.go

@@ -34,6 +34,7 @@ type keyMap struct {
 	Tab        key.Binding
 	ToggleRail key.Binding
 	Stumble    key.Binding
+	Theme      key.Binding
 }
 
 var keys = keyMap{
@@ -68,4 +69,5 @@ var keys = keyMap{
 	Tab:        key.NewBinding(key.WithKeys("tab"), key.WithHelp("tab", "focus cycle")),
 	ToggleRail: key.NewBinding(key.WithKeys("ctrl+b"), key.WithHelp("ctrl+b", "toggle tag rail")),
 	Stumble:    key.NewBinding(key.WithKeys("S"), key.WithHelp("S", "stumble")),
+	Theme:      key.NewBinding(key.WithKeys("T"), key.WithHelp("T", "theme")),
 }