add manual microphone gain and signal meter

public/index.html

@@ -138,6 +138,17 @@
     <select id="audio-input" class="settings-select">
       <option value="">System default</option>
     </select>
+    <label class="field-label" for="mic-gain">Microphone gain</label>
+    <div class="gain-control">
+      <input id="mic-gain" type="range" min="0" max="2" step="0.05" value="1" />
+      <span id="mic-gain-value">100%</span>
+    </div>
+    <div class="signal-preview" aria-live="polite">
+      <span>Signal</span>
+      <meter id="mic-level" min="0" max="1" low="0.35" high="0.8" optimum="0.55" value="0"></meter>
+      <span id="mic-level-value">-∞ dB</span>
+    </div>
+    <p id="mic-clipping" class="hint hidden">Clipping detected — reduce microphone gain.</p>
     <p class="hint">Changes apply immediately when connected, or the next time you join.</p>
     <p class="hint">Overrides the server-provided ICE configuration (stored in <code>localStorage</code>, applied on next join).</p>
     <label class="toggle-row">