feat: make FAB circular on Marmot Groups screen

Matches the CircleShape used by FABs on other screens.

https://claude.ai/code/session_013dhfy18dfuSYN8khXiLLBb

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/chats/marmotGroup/MarmotGroupListScreen.kt

@@ -30,6 +30,7 @@ import androidx.compose.foundation.layout.fillMaxWidth
 import androidx.compose.foundation.layout.padding
 import androidx.compose.foundation.lazy.LazyColumn
 import androidx.compose.foundation.lazy.items
+import androidx.compose.foundation.shape.CircleShape
 import androidx.compose.material.icons.Icons
 import androidx.compose.material.icons.automirrored.filled.ArrowBack
 import androidx.compose.material.icons.filled.Add
@@ -128,7 +129,7 @@ fun MarmotGroupListScreen(
         },
         snackbarHost = { SnackbarHost(snackbarHostState) },
         floatingActionButton = {
-            FloatingActionButton(onClick = { nav.nav(Route.CreateMarmotGroup) }) {
+            FloatingActionButton(onClick = { nav.nav(Route.CreateMarmotGroup) }, shape = CircleShape) {
                 Icon(Icons.Default.Add, contentDescription = "Create Group")
             }
         },