Clarify mic mute state, center spatial map on you, show talking/muted on shares

- Mic button now swaps glyph (mic vs muted) and syncs immediately on
  join/leave/load instead of relying on a subtle CSS underline.
- Spatial minimap camera now follows the local user, matching how the
  audio panning is already centered on them, instead of plotting
  absolute room coordinates stretched across whatever width the
  screen happens to be.
- Screen-share tiles now show a speaking glow and muted badge, kept in
  sync on the existing audio-meter tick without tearing down <video>
  elements.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

public/style.css

@@ -144,6 +144,10 @@ body {
   padding: 3px 8px; border-radius: 5px; font-size: 12px; font-weight: 600;
 }
 .screen-card .label .live { color: var(--red); font-weight: 800; margin-right: 5px; }
+.screen-card.speaking { box-shadow: 0 0 0 3px var(--green), 0 8px 24px rgba(0,0,0,.4); }
+.tile-mic-badge { margin-left: 6px; }
+.tile-mic-badge.muted::before { content: "\1F507"; } /* 🔇 */
+.tile-mic-badge:not(.muted)::before { content: ""; }
 
 /* channel user lines */
 .channel-users {