Pdf reflow rework (#51)

* perf(pdf): native Pdfium-based reflow engine with auto-open

Migrate PDF-to-Markdown reflow generation from PDFBox to a custom
native implementation using Pdfium. This transition improves
processing speed by ~10x and significantly reduces memory overhead.

- Native JNI Bridge: Implemented `pdfium_bridge.cpp` using `dlopen`
  to hook into the existing `libpdfium.so` memory space.
- Optimized Extraction: Replaced character-by-character JNI calls
  with bulk array retrieval (`getPageFontSizes`, `getPageFontWeights`),
  drastically reducing JNI boundary overhead.
- Enhanced Accuracy: Improved Markdown formatting logic by using
  native font weight (bold) and relative font size variance (headers).
- Thread Safety: Refactored generator to process pages sequentially
  while synchronized with the global `PdfiumCore.lock` to ensure
  stability across concurrent UI operations.
- Seamless UX: Implemented a reactive auto-open system in the
  PDF viewer that tracks user intent and navigates to the reflow
  view immediately upon background task completion.

* Improved PDF to Markdown conversion and added cache cleanup.

- Implemented automated cleanup of imported file caches when deleting books.
- Enhanced `PdfToMarkdownGenerator` with support for font flags (italics), improved kerning, and smarter paragraph wrapping.
- Updated `NativePdfiumBridge` and C++ JNI code to extract font information flags from PDFium.

* Implemented seamless file switching and enhanced reflow transition logic.

Key changes include:
- Added `switchToFileSeamlessly` to `MainViewModel` to handle state transitions and navigation when switching between PDF and reflowed text views.
- Updated `generateAndImportReflowFile` to support automatic opening of the generated file at a specific page/chapter.
- Integrated `NavigationEvent` and `CompletableDeferred` to manage asynchronous navigation and state updates during file switches.
- Modified `EpubReaderScreen` and `PdfViewerScreen` to pass the current position when toggling between PDF and text modes.
- Updated `AppNavigation` to move navigation logic out of the `NavHost` and added loading overlays to viewers to improve UI feedback during transitions.

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

@@ -118,7 +118,7 @@ fun DictionarySettingsDialog(
                                         color = if (useOnlineDictionary) MaterialTheme.colorScheme.onPrimaryContainer else MaterialTheme.colorScheme.onSurface
                                     )
                                     Text(
-                                        text = "Contextual definitions powered by AI.",
+                                        text = "Definitions powered by AI.",
                                         style = MaterialTheme.typography.bodySmall,
                                         color = if (useOnlineDictionary) MaterialTheme.colorScheme.onPrimaryContainer.copy(alpha = 0.8f) else MaterialTheme.colorScheme.onSurfaceVariant
                                     )