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/AppDatabase.kt

@@ -27,7 +27,7 @@ import androidx.room.TypeConverters
 import androidx.room.migration.Migration
 import androidx.sqlite.db.SupportSQLiteDatabase
 
-@Database(entities = [RecentFileEntity::class, CustomFontEntity::class], version = 12, exportSchema = false)
+@Database(entities = [RecentFileEntity::class, CustomFontEntity::class], version = 13, exportSchema = false)
 @TypeConverters(FileTypeConverter::class)
 abstract class AppDatabase : RoomDatabase() {
     abstract fun recentFileDao(): RecentFileDao
@@ -167,6 +167,12 @@ abstract class AppDatabase : RoomDatabase() {
             }
         }
 
+        val MIGRATION_12_13 = object : Migration(12, 13) {
+            override fun migrate(db: SupportSQLiteDatabase) {
+                db.execSQL("ALTER TABLE recent_files ADD COLUMN isReflowPreferred INTEGER NOT NULL DEFAULT 0")
+            }
+        }
+
         fun getDatabase(context: Context): AppDatabase {
             return INSTANCE ?: synchronized(this) {
                 val instance = Room.databaseBuilder(
@@ -174,11 +180,11 @@ abstract class AppDatabase : RoomDatabase() {
                     AppDatabase::class.java,
                     "reader_database"
                 )
-                    // 4. Add migration to builder
                     .addMigrations(
                         MIGRATION_1_2, MIGRATION_2_3, MIGRATION_3_4, MIGRATION_4_5,
                         MIGRATION_5_6, MIGRATION_6_7, MIGRATION_7_8, MIGRATION_8_9,
-                        MIGRATION_9_10, MIGRATION_10_11, MIGRATION_11_12
+                        MIGRATION_9_10, MIGRATION_10_11, MIGRATION_11_12,
+                        MIGRATION_12_13
                     )
                     .fallbackToDestructiveMigration(false)
                     .build()