feat(audio-rooms): listener counter badge on the full-screen room view

Renders "%d listeners" right under the room title once the kind-10312
aggregator has at least one entry. Counts every peer with a recent
presence event — hosts, speakers, and audience alike — matching what
the nostrnests web UI shows in its room header.

The badge uses a pluralized string resource so the wire copy matches
locale rules (one / many) without per-locale code paths.
Hidden when the count is zero so a brand-new room (before anyone's
presence has propagated) doesn't flash a misleading "0 listeners".

Counts can flicker briefly when a single peer drops a heartbeat —
the eviction window in AudioRoomActivityContent is 6 min so a short
relay hiccup doesn't flip the count, but the badge will move when
peers genuinely come and go.

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/audiorooms/room/AudioRoomFullScreen.kt

@@ -40,6 +40,7 @@ import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.OutlinedButton
 import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
+import androidx.compose.runtime.collectAsState
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.saveable.rememberSaveable
@@ -103,6 +104,25 @@ internal fun AudioRoomFullScreen(
             )
         }
 
+        // Listener counter — counts every active kind-10312 presence
+        // in the room. Hidden until the aggregator has at least one
+        // entry so the placeholder doesn't flash on entry.
+        val presences by viewModel.presences.collectAsState()
+        val listenerCount = presences.size
+        if (listenerCount > 0) {
+            Text(
+                text =
+                    androidx.compose.ui.res.pluralStringResource(
+                        R.plurals.audio_room_listener_count,
+                        listenerCount,
+                        listenerCount,
+                    ),
+                style = MaterialTheme.typography.bodySmall,
+                color = MaterialTheme.colorScheme.onSurfaceVariant,
+                modifier = Modifier.padding(top = 4.dp),
+            )
+        }
+
         if (onStage.isNotEmpty()) {
             StagePeopleRow(
                 label = stringRes(R.string.audio_room_stage),

amethyst/src/main/res/values/strings.xml

@@ -521,6 +521,10 @@
     <string name="audio_room_notification_stop">Stop</string>
     <string name="audio_room_join">Join audio room</string>
     <string name="audio_room_leave">Leave</string>
+    <plurals name="audio_room_listener_count">
+        <item quantity="one">%1$d listener</item>
+        <item quantity="other">%1$d listeners</item>
+    </plurals>
     <string name="audio_room_create_fab">Start space</string>
     <string name="audio_room_create_title">Start a new audio room</string>
     <string name="audio_room_create_field_room">Room name</string>