Pdf text reflow (#36)

* Implemented PDF reflow mode by introducing a mechanism to convert PDF content to Markdown/HTML for viewing in the EPUB reader.

Specific changes include:
- Added `PdfReflowGenerator` and `PdfToMarkdownGenerator` to handle PDF text extraction and conversion to reflowable formats.
- Updated `MainViewModel` with `toggleReflowMode` logic to switch between original PDF and reflowed views.
- Modified `RecentFileEntity` and `RecentFileDao` to persist user reflow preferences, including a Room database migration (v12 to v13).
- Updated `PdfViewerScreen` and `EpubReaderControls` to include UI options for toggling reflow mode.
- Integrated reflow preference check into the book opening workflow to automatically load the preferred view.
- Updated `AppNavigation` and `EpubReaderScreen` to support the new view switching state.

* Implemented background processing and incremental loading for PDF reflow mode.

- Added `reflowProgress` to `MainViewModel` to track and display PDF-to-Markdown conversion progress in the UI.
- Refactored `PdfToMarkdownGenerator` to generate a skeleton EPUB structure immediately while processing page content (text and images) asynchronously.
- Switched PDF text extraction to use `PDFBox` with optimized memory settings and JPEG compression for images.
- Implemented priority page processing in reflow mode, starting with the user's current page.
- Added "Clear Reflow Cache" debug option to the Home Screen.
- Enhanced `BookPaginator` to support lazy loading of chapter content from disk and improved cache hit detection.

* Refactored PDF Reflow Mode to generate standalone Markdown files instead of temporary EPUB books.

* perf(reflow): optimize PDF-to-Markdown conversion and fix viewing lag

- Re-architected PdfToMarkdownGenerator to use a single-pass stream (O(N) complexity), fixing performance bottlenecks and timeouts on large PDFs.
- Implemented "Virtual Chaptering" in SingleFileImporter for Markdown files to split content into page-level HTML files, eliminating UI lag during reading.
- Simplified ReflowWorker to delegate progress tracking and looping to the generator.
- Enhanced PdfViewerScreen with a prominent top-bar progress indicator and a completion snackbar with an "OPEN" action.

* Optimized EPUB parsing performance and fixed PDF viewer UI layout.

- Optimized `EpubParser` by implementing parallel chapter parsing using coroutines and a semaphore to limit concurrency.
- Reduced memory usage in `EpubParser` and `SingleFileImporter` by no longer storing full HTML content in memory for chapters.
- Updated `EpubXMLFileParser` to support an existing `Document` object to avoid redundant Jsoup parsing.
- Fixed an issue in `PdfViewerScreen` where the snackbar was appearing under the bottom app bar.

app/src/main/java/com/aryan/reader/epubreader/EpubReaderControls.kt

@@ -151,7 +151,8 @@ fun EpubReaderTopBar(
     onOpenTtsSettings: () -> Unit,
     onOpenDeviceVoiceSettings: () -> Unit,
     searchFocusRequester: androidx.compose.ui.focus.FocusRequester,
-    modifier: Modifier = Modifier
+    modifier: Modifier = Modifier,
+    onToggleReflow: (() -> Unit)? = null,
 ) {
     AnimatedVisibility(
         visible = isVisible,
@@ -201,6 +202,24 @@ fun EpubReaderTopBar(
                             expanded = showMoreMenu,
                             onDismissRequest = { showMoreMenu = false }
                         ) {
+                            if (onToggleReflow != null) {
+                                DropdownMenuItem(
+                                    text = { Text("View Original PDF") },
+                                    onClick = {
+                                        showMoreMenu = false
+                                        onToggleReflow()
+                                    },
+                                    leadingIcon = {
+                                        Icon(
+                                            painter = painterResource(id = R.drawable.picture_as_pdf),
+                                            contentDescription = null,
+                                            modifier = Modifier.size(20.dp)
+                                        )
+                                    }
+                                )
+                                HorizontalDivider()
+                            }
+
                             DropdownMenuItem(
                                 text = { Text("Reading Mode: Vertical") },
                                 enabled = !isTtsActive,

app/src/main/java/com/aryan/reader/epubreader/EpubReaderScreen.kt

@@ -128,6 +128,7 @@ import com.aryan.reader.BannerMessage
 import com.aryan.reader.BuildConfig
 import com.aryan.reader.CustomTopBanner
 import com.aryan.reader.DeviceVoiceSettingsSheet
+import com.aryan.reader.MainViewModel
 import com.aryan.reader.RenderMode
 import com.aryan.reader.SearchResult
 import com.aryan.reader.SummarizationResult
@@ -291,8 +292,25 @@ fun EpubReaderScreen(
     coverImagePath: String?,
     onRenderModeChange: (RenderMode) -> Unit,
     customFonts: List<CustomFontEntity>,
-    onImportFont: (Uri) -> Unit
+    onImportFont: (Uri) -> Unit,
+    viewModel: MainViewModel
 ) {
+    val uiState by viewModel.uiState.collectAsState()
+
+    val isReflowFile = uiState.selectedBookId?.endsWith("_reflow") == true
+    val originalBookId = if (isReflowFile) uiState.selectedBookId!!.removeSuffix("_reflow") else null
+
+    val onOpenOriginal: (() -> Unit)? = if (originalBookId != null) {
+        {
+            val originalItem = uiState.recentFiles.find { it.bookId == originalBookId }
+            if (originalItem != null) {
+                viewModel.onRecentFileClicked(originalItem)
+            } else {
+                viewModel.showBanner("Original PDF not found.", true)
+            }
+        }
+    } else null
+
     EpubReaderHost(
         epubBook = epubBook,
         renderMode = renderMode,
@@ -307,7 +325,8 @@ fun EpubReaderScreen(
         coverImagePath = coverImagePath,
         onRenderModeChange = onRenderModeChange,
         customFonts = customFonts,
-        onImportFont = onImportFont
+        onImportFont = onImportFont,
+        onToggleReflow = onOpenOriginal
     )
 }
 
@@ -330,7 +349,8 @@ fun EpubReaderHost(
     coverImagePath: String?,
     onRenderModeChange: (RenderMode) -> Unit,
     customFonts: List<CustomFontEntity>,
-    onImportFont: (Uri) -> Unit
+    onImportFont: (Uri) -> Unit,
+    onToggleReflow: (() -> Unit)? = null
 ) {
     val view = LocalView.current
     val context = LocalContext.current
@@ -389,7 +409,9 @@ fun EpubReaderHost(
 
     var isAutoScrollCollapsed by remember { mutableStateOf(false) }
 
-    val bookId = remember(epubBook.title) { getBookIdForPrefs(epubBook.title) }
+    val bookId = remember(epubBook.title, epubBook.fileName) {
+        if (epubBook.fileName.length > 20) epubBook.fileName else getBookIdForPrefs(epubBook.title)
+    }
     var isAutoScrollLocal by remember { mutableStateOf(loadAutoScrollLocalMode(context, bookId)) }
 
     val initialSettings = remember(isAutoScrollLocal) {
@@ -1008,6 +1030,7 @@ fun EpubReaderHost(
         chapterChunks = result.chunks
         isChapterParsing = false
         loadUpToChunkIndex = result.startChunkIndex
+        Timber.tag("ReflowPaginationDiag").d("EpubReaderScreen: loadChapterContent finished. chapterChunks.size=${chapterChunks.size}, isChapterParsing=$isChapterParsing")
 
         if (chunkTargetOverride != null) {
             chunkTargetOverride = null
@@ -1030,6 +1053,7 @@ fun EpubReaderHost(
 
     var isPagerInitialized by remember(initialLocator) { mutableStateOf(initialLocator == null) }
     LaunchedEffect(paginator, currentRenderMode, isPagerInitialized) {
+        Timber.tag("ReflowPaginationDiag").d("EpubReaderScreen: Checking paginator init. currentRenderMode=$currentRenderMode, paginator=${paginator != null}, isPagerInitialized=$isPagerInitialized")
         if (currentRenderMode == RenderMode.PAGINATED && paginator != null && !isPagerInitialized) {
             scope.launch {
                 val bookPaginator = paginator as? BookPaginator
@@ -2880,7 +2904,8 @@ fun EpubReaderHost(
                     searchFocusRequester = searchFocusRequester,
                     modifier = Modifier.align(Alignment.TopCenter),
                     onOpenTtsSettings = { showTtsSettingsSheet = true },
-                    onOpenDeviceVoiceSettings = { showDeviceVoiceSettingsSheet = true }
+                    onOpenDeviceVoiceSettings = { showDeviceVoiceSettingsSheet = true },
+                    onToggleReflow = onToggleReflow,
                 )
 
                 val autoScrollPadding by androidx.compose.animation.core.animateDpAsState(