text-align:center had no effect because display:flex makes the button text
an anonymous flex item, centered only by justify-content -- which is the
exact property broken for <button> on iOS 9. Removed flexbox from .btn
entirely and switched to text-align (horizontal) + per-variant line-height
(vertical). Single-line only, but reliable on every browser including iOS 9.

Also bumped legacy symbol sizes per feedback: <</>> 30px, ■/► 40px,
vol/mut 18px, PL 20px.

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