Rebrand as Playlink with a new sunset theme

- Name: Playlink in the UI, page title, footer, messages and README
  (file names like playlink.db and .playlink-trash are unchanged).
- Theme: plum-black surfaces with a coral-to-pink accent, a soft glow
  behind the page, gradient primary buttons, toggles and progress bars,
  accent focus rings and slim scrollbars.
- New logo (beamed notes on the brand gradient) and a Play/link wordmark.
- Inter variable font, bundled so it works under the strict CSP.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/engine/ops.go

@@ -626,7 +626,7 @@ func (e *Engine) Undo(ctx context.Context, opID int64) error {
 	default:
 		// History can hold operations of features that were removed (tag
 		// edits, file moves); undoing them as a link would be wrong.
-		return fmt.Errorf("%q operations can no longer be undone from playlink; their originals stay in the quarantine folder", o.Action)
+		return fmt.Errorf("%q operations can no longer be undone from Playlink; their originals stay in the quarantine folder", o.Action)
 	}
 	l, err := e.loadPair(ctx, o.PairID)
 	if err != nil {