Render embedded annotations in PDF (#59)

* Added support for reading and displaying embedded PDF annotations.

Changes include:
- Added JNI bindings in `NativePdfiumBridge` and `pdfium_bridge.cpp` to extract annotation count, subtypes, rectangles, and string values (Contents, NM, IRT, T) using PDFium.
- Introduced `EmbeddedAnnotation` data class to represent PDF annotations and their reply threads.
- Implemented annotation extraction logic in `PdfPageComposable` to fetch, group, and map embedded annotations (Sticky Notes, Highlights, etc.) to screen coordinates.
- Updated `PdfSelectionMenuPopup` to support displaying and copying comment threads.
- Improved selection menu positioning and hit detection for annotations.
- Added a safety check to recycle bitmaps in `PdfTile` rendering if handover to the main thread fails.

* Improved comment thread UI

app/src/main/java/com/aryan/reader/pdf/NativePdfiumBridge.kt

@@ -11,4 +11,14 @@ object NativePdfiumBridge {
     @JvmStatic external fun getPageFontSizes(textPagePtr: Long, count: Int): FloatArray?
     @JvmStatic external fun getPageFontWeights(textPagePtr: Long, count: Int): IntArray?
     @JvmStatic external fun getPageFontFlags(textPagePtr: Long, count: Int): IntArray?
+
+    @JvmStatic external fun getAnnotCount(pagePtr: Long): Int
+    @JvmStatic external fun getAnnotSubtype(pagePtr: Long, index: Int): Int
+    @JvmStatic external fun getAnnotRect(pagePtr: Long, index: Int): FloatArray?
+    @JvmStatic external fun getAnnotString(pagePtr: Long, index: Int, key: String): String?
+
+    const val ANNOT_TEXT = 1         // Sticky Note
+    const val ANNOT_LINK = 2         // Link
+    const val ANNOT_HIGHLIGHT = 8    // Highlight
+    const val ANNOT_INK = 12         // Freehand drawing
 }