feat: hide call buttons in chats with more than 5 members

Full-mesh WebRTC group calls degrade at 5+ participants (each
participant uploads N-1 streams). Hide the voice and video call
buttons in the chat header when the room has more than 5 members.

https://claude.ai/code/session_013h2E7spwHDgSjunqumsYgp

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/chats/privateDM/ChatroomScreen.kt

@@ -50,6 +50,7 @@ fun ChatroomScreen(
 ) {
     val context = LocalContext.current
     val isGroupChat = roomId.users.size > 1
+    val isCallSupported = roomId.users.size <= 5
     val startVoiceCall =
         rememberCallWithPermission(context) {
             ActiveCallHolder.set(accountViewModel.callManager, accountViewModel.callController, accountViewModel)
@@ -80,8 +81,8 @@ fun ChatroomScreen(
                 room = roomId,
                 accountViewModel = accountViewModel,
                 nav = nav,
-                onCallClick = { _ -> startVoiceCall() },
+                onCallClick = if (isCallSupported) ({ _ -> startVoiceCall() }) else null,
-                onVideoCallClick = { _ -> startVideoCall() },
+                onVideoCallClick = if (isCallSupported) ({ _ -> startVideoCall() }) else null,
             )
         },
         accountViewModel = accountViewModel,