fix(a11y): move the focus to the content after a change of page

[ronjalh]

What

Part of the peer feedback (#167): the focus is not moved to the content when going to another page, after "Load more" it ends on the body, and the sort menu seemed to stay open after a choice with the keyboard.

I checked each point first:

• Going to another page: new FocusOnNavigate in src/App.tsx puts the focus on #main-content (the skip link target) when the path changes. A change of the search, filters or sort is not a new page, and nothing moves on the first load.
• The sort menu: it already closes after a choice and gives the focus back to its button, which names the new sort ("Sort by Name: A to Z"). No code change; an end-to-end test keeps it so.
• After "Load more": not done here. "Load more" is being replaced by pagination in feat: replace Load more with pagination on the Elements page #162 (PR feat: implement pagination on Elements page #189), which changes the button and where the focus should go. The focus after a page change belongs in that PR, and this one would only conflict with it.

Checked

• Unit tests for FocusOnNavigate (it moves after a change of page, not after a change of the search). I broke the code on purpose to see the first one fail.
• End-to-end: e2e/focus.e2e.ts (2 tests) passes. All frontend tests, tsc, ESLint and Prettier are clean.
• Not tested with a screen reader.

Refs: #172 (the Load more part is left open, so this does not close it)

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