audio: replace ♪̸ toggle glyphs with 🔇/🔊 emoji

The combining solidus overlay (U+0338) rendered misaligned on Windows
Chrome with IBM Plex Mono — the strikethrough appeared offset from the
note glyph and looked like a rendering artifact rather than a mute icon.

Replaced with 🔇 (muted) / 🔊 (unmuted) in all four locations: base.html
initial HTML, ambient.js, synth_ambient.js, and lobby_ambient.js.

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

app/web/static/js/lobby_ambient.js

@@ -113,7 +113,7 @@
       if (toggle) {
         toggle.dataset.muted = "false";
         toggle.setAttribute("aria-pressed", "true");
-        toggle.innerHTML = '<span aria-hidden="true">♪</span>';
+        toggle.innerHTML = '<span aria-hidden="true">🔊</span>';
       }
     }
     if (!savedMuted()) { attemptPlay(); startAccents(); }

app/web/static/js/synth_ambient.js

@@ -65,8 +65,8 @@
     toggle.setAttribute("aria-pressed", String(!muted));
     toggle.title = muted ? "Unmute room audio" : "Mute room audio";
     toggle.innerHTML = muted
-      ? '<span aria-hidden="true">♪̸</span>'
-      : '<span aria-hidden="true">♪</span>';
+      ? '<span aria-hidden="true">🔇</span>'
+      : '<span aria-hidden="true">🔊</span>';
   }
 
   function ensureStarted() {

app/web/templates/base.html

@@ -393,7 +393,7 @@
       </a>
       <nav class="text-[13px] text-[var(--mp-ink-muted)] space-x-5 flex items-center">
         <button id="mp-ambient-toggle" class="mp-ambient-toggle" data-muted="true" aria-pressed="false" aria-label="Toggle room audio">
-          <span aria-hidden="true">♪̸</span>
+          <span aria-hidden="true">🔇</span>
         </button>
         {% if player %}
         <details class="mp-user-menu">