Add single-page view to the PDF viewer

[koppor]

🤖 Reviewers used to other PDF readers can now switch to a single-page view: one page fitted to the pane, with the mouse wheel flipping to the next or previous page. A zoomed-in page scrolls within itself before flipping, and the setting is remembered.

Test: open a multi-page paper, click ▯ in the PDF toolbar and scroll the wheel down and up.
Test: zoom in while in single-page view and scroll; the page scrolls before flipping.

🤖 Generated with Claude Code

https://claude.ai/code/session_01Vzuf44Kh6QFFJtEKoMgug7

[Gram21]

Thanks again for your contribution!

In this PR, I am a bit torn. I think the idea can be great (although I usually do not use such a mode).
However, I think there are some usability issues in my opinion.

1. the icon on the button is a bit weird and not immediately recognizable what it is doing. At the same time it has no hover-text explaining it (some other buttons as well, I recognized; we need to add more in the future).
2. It can be just me, but the overall handling feels clunky, at least on a trackpad. This might also be some bug where a zoomed-in page looks not zoomed in but behaves like it. We need to look closer into that.

So, to sum up, the feature is great but we need to change a few things before we can merge the PR.