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/backup/backup.go

@@ -41,7 +41,7 @@ const (
 
 var nameRe = regexp.MustCompile(`^playlink_backup_(manual|scheduled|action)_\d{4}\.\d{2}\.\d{2}_\d{2}\.\d{2}\.\d{2}(_\d+)?\.zip$`)
 
-var ErrBadName = errors.New("not a playlink backup file name")
+var ErrBadName = errors.New("not a Playlink backup file name")
 
 // Settings stored in the settings table.
 const (
@@ -341,7 +341,7 @@ func (m *Manager) Stage(ctx context.Context, zipPath string, maxSchema int) erro
 		}
 	}
 	if dbf == nil {
-		return errors.New("the zip does not contain a playlink database")
+		return errors.New("the zip does not contain a Playlink database")
 	}
 	tmp := filepath.Join(m.dataDir, stagedName+".tmp")
 	_ = os.Remove(tmp)
@@ -395,14 +395,14 @@ func validate(ctx context.Context, path string, maxSchema int) error {
 	}
 	var schema int
 	if err := db.QueryRowContext(ctx, `SELECT COALESCE(MAX(v), 0) FROM schema_version`).Scan(&schema); err != nil {
-		return errors.New("the backup is not a playlink database")
+		return errors.New("the backup is not a Playlink database")
 	}
 	if schema < 1 || schema > maxSchema {
-		return fmt.Errorf("the backup is from a newer version of playlink (schema %d, this version supports up to %d)", schema, maxSchema)
+		return fmt.Errorf("the backup is from a newer version of Playlink (schema %d, this version supports up to %d)", schema, maxSchema)
 	}
 	var n int
 	if err := db.QueryRowContext(ctx, `SELECT COUNT(*) FROM sqlite_master WHERE type = 'table' AND name IN ('pairs', 'users', 'operations')`).Scan(&n); err != nil || n != 3 {
-		return errors.New("the backup is not a playlink database")
+		return errors.New("the backup is not a Playlink database")
 	}
 	return nil
 }