feat(broadcast): float banner as a rounded card

The broadcast banner was rendered full-width with a 50dp bottom
offset that assumed a bottom navigation bar underneath. On screens
without the bottom bar the flat rectangle hovered above empty space,
which looked like a misaligned attachment rather than an overlay.

Restyle the banner as a floating card: 94% width (capped at 560dp for
tablets), rounded corners, shadow elevation. Keep it above a bottom
bar when present (50dp bar + 8dp float gap), but since it now reads
as an intentional floating toast, the same offset looks correct on
screens without a bottom bar as well.

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/broadcast/BroadcastBanner.kt

@@ -38,6 +38,7 @@ import androidx.compose.foundation.layout.height
 import androidx.compose.foundation.layout.padding
 import androidx.compose.foundation.layout.size
 import androidx.compose.foundation.layout.width
+import androidx.compose.foundation.shape.RoundedCornerShape
 import androidx.compose.material.icons.Icons
 import androidx.compose.material.icons.filled.CheckCircle
 import androidx.compose.material.icons.filled.Error
@@ -97,7 +98,9 @@ fun BroadcastBanner(
     ) {
         Surface(
             color = MaterialTheme.colorScheme.surfaceContainer,
-            tonalElevation = 2.dp,
+            tonalElevation = 3.dp,
+            shadowElevation = 6.dp,
+            shape = RoundedCornerShape(20.dp),
             modifier =
                 Modifier
                     .fillMaxWidth()

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/broadcast/DisplayBroadcastProgress.kt

@@ -25,6 +25,7 @@ import androidx.compose.foundation.layout.fillMaxSize
 import androidx.compose.foundation.layout.fillMaxWidth
 import androidx.compose.foundation.layout.navigationBarsPadding
 import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.layout.widthIn
 import androidx.compose.material3.ExperimentalMaterial3Api
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.LaunchedEffect
@@ -130,10 +131,11 @@ fun DisplaySnack(
             },
             modifier =
                 Modifier
-                    .fillMaxWidth()
                     .align(Alignment.BottomCenter)
                     .navigationBarsPadding()
-                    .padding(bottom = 50.dp),
+                    .padding(start = 12.dp, end = 12.dp, bottom = 58.dp)
+                    .fillMaxWidth(0.94f)
+                    .widthIn(max = 560.dp),
         )
     }
 }