Guest page: hear yourself, clearer mute, 'What is happening' log

Ideas taken from CurryCaster's guest page:
- 'Hear yourself' plays the guest's own mic in their headphones
  (refused when the headphones box is unticked - it would feed back)
- Mute shows a clear 'the show cannot hear you' state
- 'What is happening' log: connection events plus stats every 15 s in
  both directions (kb/s, loss, jitter, buffer, concealment, round trip,
  candidate types). Never includes IP addresses. Kept in the page only;
  the guest presses Copy and sends it to the host.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/static/style.css

@@ -35,3 +35,6 @@ button.small{padding:3px 10px;font-size:13px}
 .bar i{display:block;height:100%;width:0;background:var(--ok);transition:width .08s linear}
 .bar i.hot{background:var(--warn)}.bar i.clip{background:var(--danger)}
 .meter b{font-weight:400;color:var(--muted);text-align:right;font-variant-numeric:tabular-nums}
+.diag{margin-top:14px;font-size:14px}.diag summary{cursor:pointer}
+.diaglog{max-height:240px;overflow:auto;background:var(--bg);border:1px solid var(--line);border-radius:7px;padding:8px;font-size:12px;white-space:pre-wrap;overflow-wrap:anywhere;margin:0}
+button.on{border-color:var(--ok);color:var(--ok)}