fix for the "jump back" bug that occurred when scrolling up in vertical continuous and webtoon reader modes.
- `ChapterPreloadManager` was maintaining its own `_currentIndex`.
- When prepending previous chapter pages, the index was being incremented
twice (once in the manager + once in `_handlePrevChapterPrepended`).
- This caused `itemScrollController.jumpTo()` to overshoot, resulting in
a visible jump forward (perceived as "jump back" while trying to scroll up).
- Removed all index management (`_currentIndex`, getter, setter, startIndex)
from `ChapterPreloadManager` and `ReaderMemoryManagement`.
- `ChapterPreloadManager` is now a pure data container (only manages `_pages`).
- `_handlePrevChapterPrepended` now captures the **current visible top index**
*before* prepending and adjusts the scroll position only once.
- `_readProgressListener` is now the single source of truth for `_currentIndex`.
- Removed stale `initialScrollIndex` logic from preload initialization.
`ChapterPreloadManager.preloadPrevChapter` does `_currentIndex += prependCount` (internal manager index).
Then `_handlePrevChapterPrepended` in `reader_view.dart` does the exact same thing again to the UI’s `_currentIndex` before calling `jumpTo`.
The UI state already handles the adjustment + `jumpTo` correctly.
The manager’s internal `_currentIndex` is not needed for continuous mode (the `ItemPositionsListener` overrides it anyway).
- PageIndicator widget to display current page and total pages.
- Created ReaderAppBar for navigation and chapter information.
- ReaderBottomBar for page navigation and settings access.
- Added ReaderGestureHandler for managing tap zones and gestures.
- ReaderSettingsModal for user-configurable settings.
