fix(ui): size legacy iOS 9 symbols to fill ~3/4 of button height

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>

web/static/app.js

@@ -256,6 +256,7 @@ function updateVolumeFill(muted) {
 }
 function updateMuteBtn(muted) {
   btnMute.textContent = muted ? SYM.volOff : SYM.volOn;
+  if (isLegacyIOS) { btnMute.style.fontSize = '16px'; btnMute.style.fontWeight = 'bold'; }
   btnMute.classList.toggle('muted', muted);
 }
 
@@ -560,14 +561,33 @@ function scrollToCurrentTrack() {
 // ── Boot ──────────────────────────────────────────────────────────────────────
 
 // Replace emoji with legacy symbols on iOS 9 where the glyphs are missing.
+// Centering is already handled by flex on .btn — we only set size/weight here.
 if (isLegacyIOS) {
-  $('btn-prev').textContent         = SYM.prev;
-  $('btn-play').textContent         = SYM.play;
-  $('btn-stop').textContent         = SYM.stop;
-  $('btn-next').textContent         = SYM.next;
-  $('btn-mute').textContent         = SYM.volOn;
-  $('btn-show-playlist').textContent = SYM.playlist;
-  $('btn-kill').textContent         = SYM.skip + ' Überspringen';
+  // Helper: replace text and tune font so the symbol fills ~¾ of the button.
+  function legacySym(id, text, size, weight) {
+    var el = $(id);
+    el.textContent = text;
+    el.style.fontSize   = size;
+    el.style.fontWeight = weight || 'bold';
+    el.style.letterSpacing = '2px';
+  }
+
+  // Transport controls (btn-h = 64px):
+  //   two-char symbols (<<, >>) → 26px bold; single-char (■) → 36px bold.
+  //   ▶ is natively supported on iOS 9 and already 32px via .btn-play.
+  legacySym('btn-prev', SYM.prev, '26px');   // <<
+  legacySym('btn-play', SYM.play, '36px');   // ▶
+  legacySym('btn-stop', SYM.stop, '36px');   // ■
+  legacySym('btn-next', SYM.next, '26px');   // >>
+
+  // Volume mute (48px tall): short text, large and bold
+  legacySym('btn-mute', SYM.volOn, '16px');
+
+  // Playlist action button (52px tall)
+  legacySym('btn-show-playlist', SYM.playlist, '18px');
+
+  // Kill button: just drop the emoji, keep the label text
+  $('btn-kill').textContent = SYM.skip + ' Überspringen';
 }
 
 connect();