Epub improvements (#160)

* Improved navigation and search result highlighting in the EPUB reader

* refactor: implement JIT chunk restoration for robust navigation

- Fixes search and bookmark navigation failing in same-chapter transitions.
- Introduced JIT (Just-in-Time) HTML restoration in `epub_reader.js` to
  re-populate virtualized chunks before CFI resolution or search scrolling.
- Synchronized search highlighting and user highlights within
  newly-restored chunk segments.
- Improved search navigation accuracy by mapping occurrences to
  relative chunk indices.

* Implemented a custom text selection engine for the paginated EPUB reader to support cross-page selection and improved handle interaction.

* Implemented management of external file behavior and improved library filtering.

* Added support for toolbar customization in the EPUB reader.

* Added support for toolbar customization in the PDF reader.

* fix: rendering during auto-scroll and navigation on long pages

* Added a "Scroll to Top" feature to the auto-scroll controls in both EPUB and PDF readers.

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

@@ -420,7 +420,7 @@ class PdfRichTextRepository(private val context: Context) {
                             isItalic = sObj.optBoolean("i"),
                             isUnderline = sObj.optBoolean("u"),
                             isStrikethrough = sObj.optBoolean("st"),
-                            fontPath = sObj.optString("fp", null),
+                            fontPath = sObj.optString("fp"),
                         )
                     )
                 }