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/PdfVerticalReader.kt

@@ -139,6 +139,8 @@ class VerticalPdfReaderState {
     internal var scrollToPageHandler: (suspend (Int) -> Unit)? = null
     internal var snapToPageHandler: (suspend (Int) -> Unit)? = null
     internal var scrollByHandler: (suspend (Float) -> Unit)? = null
+    internal var scrollToTopHandler: (suspend () -> Unit)? = null
+    internal var scrollToBottomHandler: (suspend () -> Unit)? = null
 
     suspend fun scrollToPage(pageIndex: Int) {
         scrollToPageHandler?.invoke(pageIndex)
@@ -151,6 +153,14 @@ class VerticalPdfReaderState {
     suspend fun scrollBy(delta: Float) {
         scrollByHandler?.invoke(delta)
     }
+
+    suspend fun scrollToTop() {
+        scrollToTopHandler?.invoke()
+    }
+
+    suspend fun scrollToBottom() {
+        scrollToBottomHandler?.invoke()
+    }
 }
 
 @Composable
@@ -244,6 +254,8 @@ internal fun PdfVerticalReader(
             state.scrollToPageHandler = null
             state.snapToPageHandler = null
             state.scrollByHandler = null
+            state.scrollToTopHandler = null
+            state.scrollToBottomHandler = null
         }
     }
     var globalEraserPosition by remember { mutableStateOf<Offset?>(null) }
@@ -527,6 +539,23 @@ internal fun PdfVerticalReader(
                     )
                 }
             }
+
+            state.scrollToTopHandler = {
+                panYAnimatable.animateTo(
+                    targetValue = headerHeightPx,
+                    animationSpec = tween(durationMillis = 500, easing = FastOutSlowInEasing)
+                )
+            }
+
+            state.scrollToBottomHandler = {
+                val currentZoom = zoomAnimatable.value
+                val zoomedDocHeight = totalDocHeight * currentZoom
+                val minPanY = (screenHeight - footerHeightPx - zoomedDocHeight).coerceAtMost(headerHeightPx)
+                panYAnimatable.animateTo(
+                    targetValue = minPanY,
+                    animationSpec = tween(durationMillis = 500, easing = FastOutSlowInEasing)
+                )
+            }
         }
 
         var selectionClearTrigger by remember { mutableLongStateOf(0L) }