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/cover.go

@@ -458,8 +458,8 @@ func (c *coverView) build(cols, rows int) []styledCell {
 			}
 			ch, fg, bg := st.bestCell(px)
 			cells = append(cells, styledCell{ch,
-				tcell.NewRGBColor(int32(fg.R), int32(fg.G), int32(fg.B)),
-				tcell.NewRGBColor(int32(bg.R), int32(bg.G), int32(bg.B))})
+				notToken(tcell.NewRGBColor(int32(fg.R), int32(fg.G), int32(fg.B))),
+				notToken(tcell.NewRGBColor(int32(bg.R), int32(bg.G), int32(bg.B)))})
 		}
 	}
 	return cells