feat(android): artist fast-scroll index bar with letter bubble

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

android/app/src/main/kotlin/com/novasonic/app/ui/screen/LibraryScreen.kt

@@ -9,6 +9,7 @@ import androidx.compose.foundation.lazy.grid.LazyVerticalGrid
 import androidx.compose.foundation.lazy.grid.items
 import androidx.compose.foundation.lazy.grid.rememberLazyGridState
 import androidx.compose.foundation.lazy.items
+import androidx.compose.foundation.lazy.rememberLazyListState
 import androidx.compose.foundation.shape.CircleShape
 import androidx.compose.foundation.shape.RoundedCornerShape
 import androidx.compose.foundation.border
@@ -20,9 +21,12 @@ import androidx.compose.runtime.*
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.draw.clip
+import androidx.compose.ui.input.pointer.PointerEventType
+import androidx.compose.ui.input.pointer.pointerInput
 import androidx.compose.ui.layout.ContentScale
 import androidx.compose.ui.text.font.FontWeight
 import androidx.compose.ui.text.input.ImeAction
+import androidx.compose.ui.text.style.TextAlign
 import androidx.compose.ui.text.style.TextOverflow
 import androidx.compose.ui.unit.dp
 import androidx.compose.ui.unit.sp
@@ -37,6 +41,7 @@ import com.novasonic.app.ui.theme.*
 import com.novasonic.app.viewmodel.AlbumSort
 import com.novasonic.app.viewmodel.LibraryTab
 import com.novasonic.app.viewmodel.LibraryViewModel
+import kotlinx.coroutines.launch
 
 @Composable
 fun LibraryScreen(
@@ -145,8 +150,26 @@ fun LibraryScreen(
                 }
             }
 
-            LibraryTab.ARTISTS -> LazyColumn(
-                contentPadding = PaddingValues(vertical = 8.dp),
+            LibraryTab.ARTISTS -> {
+                val listState  = rememberLazyListState()
+                val scope      = rememberCoroutineScope()
+                val letters    = remember(state.artistIndexes) { state.artistIndexes.map { it.name } }
+                // Map letter → flat LazyColumn index of its header item
+                val letterToIndex = remember(state.artistIndexes) {
+                    buildMap<String, Int> {
+                        var flat = 0
+                        state.artistIndexes.forEach { idx ->
+                            put(idx.name, flat)
+                            flat += 1 + idx.artist.size
+                        }
+                    }
+                }
+                var activeLetter by remember { mutableStateOf<String?>(null) }
+
+                Box(Modifier.fillMaxSize()) {
+                    LazyColumn(
+                        state = listState,
+                        contentPadding = PaddingValues(top = 8.dp, bottom = 8.dp, end = 28.dp),
                         modifier = Modifier.fillMaxSize(),
                     ) {
                         state.artistIndexes.forEach { idx ->
@@ -186,6 +209,78 @@ fun LibraryScreen(
                         }
                     }
 
+                    if (letters.isNotEmpty()) {
+                        // Bubble showing active letter while dragging
+                        activeLetter?.let { letter ->
+                            Box(
+                                contentAlignment = Alignment.Center,
+                                modifier = Modifier
+                                    .align(Alignment.CenterEnd)
+                                    .offset(x = (-44).dp)
+                                    .size(52.dp)
+                                    .clip(CircleShape)
+                                    .background(NsAccent),
+                            ) {
+                                Text(
+                                    text = letter,
+                                    fontSize = 22.sp,
+                                    fontWeight = FontWeight.Bold,
+                                    color = NsBackground,
+                                )
+                            }
+                        }
+
+                        // Vertical letter strip
+                        Box(
+                            modifier = Modifier
+                                .align(Alignment.CenterEnd)
+                                .fillMaxHeight()
+                                .width(28.dp)
+                                .pointerInput(letters) {
+                                    awaitPointerEventScope {
+                                        while (true) {
+                                            val event = awaitPointerEvent()
+                                            val y = event.changes.firstOrNull()?.position?.y ?: continue
+                                            val idx = ((y / size.height) * letters.size)
+                                                .toInt().coerceIn(0, letters.lastIndex)
+                                            val letter = letters[idx]
+                                            when (event.type) {
+                                                PointerEventType.Press,
+                                                PointerEventType.Move -> {
+                                                    event.changes.forEach { it.consume() }
+                                                    if (letter != activeLetter) {
+                                                        activeLetter = letter
+                                                        letterToIndex[letter]?.let { pos ->
+                                                            scope.launch { listState.scrollToItem(pos) }
+                                                        }
+                                                    }
+                                                }
+                                                else -> activeLetter = null
+                                            }
+                                        }
+                                    }
+                                },
+                        ) {
+                            Column(
+                                verticalArrangement   = Arrangement.SpaceEvenly,
+                                horizontalAlignment   = Alignment.CenterHorizontally,
+                                modifier = Modifier.fillMaxSize(),
+                            ) {
+                                letters.forEach { letter ->
+                                    Text(
+                                        text       = letter,
+                                        fontSize   = 10.sp,
+                                        fontWeight = if (letter == activeLetter) FontWeight.Bold else FontWeight.Normal,
+                                        color      = if (letter == activeLetter) NsAccent else NsOnSurface3,
+                                        textAlign  = TextAlign.Center,
+                                    )
+                                }
+                            }
+                        }
+                    }
+                }
+            }
+
             LibraryTab.PLAYLISTS -> LazyVerticalGrid(
                 columns = GridCells.Adaptive(minSize = 150.dp),
                 contentPadding = PaddingValues(start = 16.dp, end = 16.dp, top = 16.dp, bottom = 80.dp),