Switch the theme live

Widgets copy their colors when they are created, so they now get role
tokens (one unique RGB value per role) instead of colors, and a wrapper
around the tcell screen replaces the tokens with the current theme's
colors in SetContent. Changing the theme in the Config view swaps that
table; the next draw recolors everything. Poster colors that equal a
token are nudged off it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019reK4DUGYqDUjniWYJvwUW

internal/ui/notify.go

@@ -144,7 +144,7 @@ func (r *toastRoot) Draw(screen tcell.Screen) {
 		if t.expires.Sub(now) < toastFade {
 			fg = named("gray") // fade out
 		}
-		style := tcell.StyleDefault.Background(current.toast).Foreground(fg)
+		style := tcell.StyleDefault.Background(tokens.toast).Foreground(fg)
 		for cx := x; cx < x+width; cx++ {
 			screen.SetContent(cx, y, ' ', nil, style)
 		}