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/data/RecentFileEntity.kt

@@ -47,5 +47,6 @@ data class RecentFileEntity(
     val locatorBlockIndex: Int?,
     val locatorCharOffset: Int?,
     val bookmarks: String?,
-    @ColumnInfo(defaultValue = "NULL") val sourceFolderUri: String?
+    @ColumnInfo(defaultValue = "NULL") val sourceFolderUri: String?,
+    @ColumnInfo(defaultValue = "0") val isReflowPreferred: Boolean
 )