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/guest.html

@@ -15,13 +15,21 @@
     <label class="check"><input type="checkbox" id="headphones" checked> I'm wearing headphones <span class="muted">(best quality; untick only if you must use speakers)</span></label>
     <div class="row">
       <button id="join" class="primary">Join the show</button>
-      <button id="mute" class="hidden">Mute mic</button>
+      <button id="mute" class="hidden">Mute my mic</button>
+      <button id="hearself" class="hidden" title="Play your own microphone in your headphones">Hear yourself</button>
       <button id="leave" class="hidden danger">Leave</button>
     </div>
     <div id="callstate" class="muted">Not connected to audio.</div>
     <div class="meter"><span>Your mic</span><div class="bar"><i id="mMic"></i></div></div>
     <div class="meter"><span>The show</span><div class="bar"><i id="mShow"></i></div></div>
     <audio id="player" autoplay></audio>
+    <details id="diag" class="diag">
+      <summary>What is happening <span class="muted small">(for troubleshooting)</span></summary>
+      <p class="muted small">If something sounds wrong, press Copy and paste this into the chat or an email to the host.
+        It shows connection quality in both directions. It never includes your IP address.</p>
+      <div class="row"><button id="copylog" class="small">Copy</button><span id="copied" class="muted small"></span></div>
+      <pre id="diaglog" class="diaglog"></pre>
+    </details>
   </section>
   <section class="card chat">
     <h2>Chat with the host</h2>