feat(phase5): reader engine integration — Checkpoint model + ChapterDrawer

- Checkpoint: serializable model for reading position (bookId, chapterIndex, pageIndex, scrollOffset, progress, timestamp).
- ChapterDrawer: ModalBottomSheet with chapter list, current chapter indicator, progress percentage.
- Volume keys and chapter navigation already present in EpubReaderScreen/PdfViewerScreen.
- ':app:assembleOssDebug' passes.

app/src/main/java/org/dueattendant149/bookreader/reader/ChapterDrawer.kt

@@ -0,0 +1,95 @@
+package org.dueattendant149.bookreader.reader
+
+import androidx.compose.foundation.clickable
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.Spacer
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.height
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.layout.width
+import androidx.compose.foundation.lazy.LazyColumn
+import androidx.compose.foundation.lazy.itemsIndexed
+import androidx.compose.material.icons.Icons
+import androidx.compose.material.icons.automirrored.filled.ArrowBack
+import androidx.compose.material3.ExperimentalMaterial3Api
+import androidx.compose.material3.Icon
+import androidx.compose.material3.IconButton
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.ModalBottomSheet
+import androidx.compose.material3.Text
+import androidx.compose.material3.TopAppBar
+import androidx.compose.material3.rememberModalBottomSheetState
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.text.style.TextOverflow
+import androidx.compose.ui.unit.dp
+
+data class ChapterItem(
+    val title: String,
+    val pageIndex: Int,
+    val progress: Float = 0f,
+)
+
+@OptIn(ExperimentalMaterial3Api::class)
+@Composable
+fun ChapterDrawer(
+    chapters: List<ChapterItem>,
+    currentChapterIndex: Int,
+    onChapterSelected: (Int) -> Unit,
+    onDismiss: () -> Unit,
+) {
+    val sheetState = rememberModalBottomSheetState(skipPartiallyExpanded = true)
+
+    ModalBottomSheet(
+        onDismissRequest = onDismiss,
+        sheetState = sheetState,
+    ) {
+        Column(modifier = Modifier.padding(16.dp)) {
+            Text(
+                text = "Chapters",
+                style = MaterialTheme.typography.titleLarge,
+                modifier = Modifier.padding(bottom = 12.dp)
+            )
+
+            LazyColumn {
+                itemsIndexed(chapters) { index, chapter ->
+                    val isCurrent = index == currentChapterIndex
+                    Row(
+                        modifier = Modifier
+                            .fillMaxWidth()
+                            .clickable { onChapterSelected(index) }
+                            .padding(vertical = 12.dp, horizontal = 4.dp),
+                        verticalAlignment = Alignment.CenterVertically
+                    ) {
+                        Column(modifier = Modifier.weight(1f)) {
+                            Text(
+                                text = chapter.title,
+                                style = if (isCurrent) MaterialTheme.typography.titleMedium
+                                else MaterialTheme.typography.bodyMedium,
+                                color = if (isCurrent) MaterialTheme.colorScheme.primary
+                                else MaterialTheme.colorScheme.onSurface,
+                                maxLines = 1,
+                                overflow = TextOverflow.Ellipsis
+                            )
+                            Text(
+                                text = "${(chapter.progress * 100).toInt()}%",
+                                style = MaterialTheme.typography.bodySmall,
+                                color = MaterialTheme.colorScheme.onSurfaceVariant
+                            )
+                        }
+                        if (isCurrent) {
+                            Spacer(modifier = Modifier.width(8.dp))
+                            Text(
+                                text = "●",
+                                color = MaterialTheme.colorScheme.primary,
+                                style = MaterialTheme.typography.bodySmall
+                            )
+                        }
+                    }
+                }
+            }
+        }
+    }
+}

app/src/main/java/org/dueattendant149/bookreader/reader/Checkpoint.kt

@@ -0,0 +1,13 @@
+package org.dueattendant149.bookreader.reader
+
+import kotlinx.serialization.Serializable
+
+@Serializable
+data class Checkpoint(
+    val bookId: String,
+    val chapterIndex: Int = 0,
+    val pageIndex: Int = 0,
+    val scrollOffset: Int = 0,
+    val progress: Float = 0f,
+    val timestamp: Long = System.currentTimeMillis(),
+)