fix: f in the article view favourites without paging down

[thebanri]

Following up on #201: this is one of the fixes from it, on Bubble Tea.

In the article view, f favourites the article, and then the same key goes on to the viewport. bubbles' default viewport key map binds f to page down, so favouriting also jumped a page.

This drops f from the viewport's page down binding. Page down stays on pgdn and space, and the help line says so. Nothing else in the viewport key map collides with nom's keys (h, l, g, G, m, o, q).

Before / after: in a long article, press f. On master the screen lands on paragraph 10. With this change it stays at the title. Both mark the article as a favourite. Checked by running nom in a pty against a local feed. TestFavouriteInArticleDoesNotPageDown fails on master, and TestSpaceStillPagesDownInArticle checks that space still works.

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