feat(#130): add scroll-to-bottom FAB that appears when user scrolls up (#163)

Co-authored-by: M57 <hy4ri@users.noreply.github.com>

app/src/main/java/com/m57/hermescontrol/ui/chat/ChatScreen.kt

@@ -13,6 +13,8 @@ import androidx.compose.animation.core.rememberInfiniteTransition
 import androidx.compose.animation.core.tween
 import androidx.compose.animation.fadeIn
 import androidx.compose.animation.fadeOut
+import androidx.compose.animation.scaleIn
+import androidx.compose.animation.scaleOut
 import androidx.compose.animation.slideInVertically
 import androidx.compose.animation.slideOutVertically
 import androidx.compose.foundation.BorderStroke
@@ -59,6 +61,7 @@ import androidx.compose.material3.CircularProgressIndicator
 import androidx.compose.material3.DropdownMenu
 import androidx.compose.material3.DropdownMenuItem
 import androidx.compose.material3.FilledTonalButton
+import androidx.compose.material3.FloatingActionButton
 import androidx.compose.material3.HorizontalDivider
 import androidx.compose.material3.Icon
 import androidx.compose.material3.IconButton
@@ -73,6 +76,7 @@ import androidx.compose.material3.TextButton
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.DisposableEffect
 import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.derivedStateOf
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
@@ -109,6 +113,11 @@ fun ChatScreen(
 ) {
     val state by viewModel.uiState.collectAsStateWithLifecycle()
     val listState = rememberLazyListState()
+    val showScrollToBottom by remember {
+        derivedStateOf {
+            state.messages.isNotEmpty() && listState.canScrollForward
+        }
+    }
     var inputText by rememberSaveable { mutableStateOf("") }
     val snackbarHostState = remember { SnackbarHostState() }
     val isDark = isSystemInDarkTheme()
@@ -522,6 +531,31 @@ fun ChatScreen(
                         }
                     }
                 }
+
+                // Scroll-to-bottom FAB
+                AnimatedVisibility(
+                    visible = showScrollToBottom,
+                    enter = fadeIn() + scaleIn(),
+                    exit = fadeOut() + scaleOut(),
+                    modifier =
+                        Modifier
+                            .align(Alignment.BottomEnd)
+                            .padding(end = 16.dp, bottom = 16.dp),
+                ) {
+                    FloatingActionButton(
+                        onClick = {
+                            listState.animateScrollToItem(state.messages.lastIndex)
+                        },
+                        modifier = Modifier.size(40.dp),
+                        containerColor = MaterialTheme.colorScheme.surfaceContainer,
+                        contentColor = MaterialTheme.colorScheme.onSurface,
+                    ) {
+                        Icon(
+                            imageVector = Icons.Filled.KeyboardArrowDown,
+                            contentDescription = "Scroll to bottom",
+                        )
+                    }
+                }
             }
 
             ChatInputBar(