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/cpp/CMakeLists.txt

@@ -65,12 +65,12 @@ set_target_properties(mobi PROPERTIES C_VISIBILITY_PRESET default)
 # FINAL NATIVE LIBRARY FOR THE APP
 # ===================================================================
 # 6. Define our final JNI wrapper library.
-#    This single .so file will be loaded by the Android app.
 add_library(
         native-lib
         SHARED
         Woff2Converter.cpp
-        mobi_jni_bridge.c  # The placeholder file you created
+        mobi_jni_bridge.c
+        pdfium_bridge.cpp  # Add this new file
 )
 
 # 7. Tell our library where to find all necessary header files.
@@ -88,8 +88,9 @@ find_library(z-lib z)
 target_link_libraries(
         native-lib
         PRIVATE
-        woff2dec      # From woff2
-        mobi          # From libmobi
+        woff2dec
+        mobi
         ${log-lib}
-        ${z-lib}      # libmobi requires zlib
+        ${z-lib}
+        dl
 )