add manual microphone gain and signal meter

public/style.css

@@ -402,6 +402,10 @@ body.sidebar-collapsed #btn-sidebar-expand { display: grid; }
   border-radius: 6px; padding: 9px 10px; margin-bottom: 4px; font: inherit;
 }
 .settings-select:focus { outline: none; border-color: var(--brand); }
+.gain-control, .signal-preview { display: flex; align-items: center; gap: 10px; margin: 6px 0 10px; }
+.gain-control input { flex: 1; accent-color: var(--brand); }
+.gain-control span, .signal-preview > span:last-child { min-width: 48px; text-align: right; font-variant-numeric: tabular-nums; }
+.signal-preview meter { flex: 1; height: 10px; }
 #ice-json {
   width: 100%; background: var(--bg-0); color: var(--text-0); border: 1px solid var(--bg-4);
   border-radius: 6px; padding: 10px; font: 12px/1.5 ui-monospace, "Cascadia Code", monospace;