PDF easy scroll/page swipe

[Originally suggested in the discussion linked in the parent issue]

Scrolling forward by tapping on the screen once.

- Maybe like this: tapping once to go forward half a page, tapping twice to go back half a page.

- Some apps also do this: The screen is divided in half, the upper part is the upper half of the first page, the lower part is the lower half of the fist page. When you tap, the upper part changes to the next page while the lower part stays the same. When you tap a second time, the lower part also changes to the second page. Like that, you can continue playing without having to adjust your eyes.

We could have other ideas pitched in to see what's most useful.

No files or tests are named in the issue. Start by locating the Flutter screen/widget that renders PDFs and handles tap/swipe interactions, then trace where page index changes and where scroll position is controlled. Add logic for one-tap half-page progression and reverse on second tap while keeping current page segments in view as described. Verify by running the app and manually testing the PDF viewer flow, or any PDF-related widget test if present, until the tap behavior matches the requested gesture flow.

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