Two-char symbols (<< >>) get 26px bold, single-char (■ ▶) get 36px bold.
letter-spacing: 2px spreads the chars slightly for better visual weight.
Mute button (vol/mut) gets 16px bold to stay readable in its 48px container.
Flex centering already guaranteed by .btn layout -- no extra work needed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>