feat(a11y): add a go-to-top link to every page

[ronjalh]

Add a go-to-top link to every page

Closes #165

A link at the end of every page that takes a keyboard or screen reader user back to the start of the page, so they do not have to go back through everything on a long page.

How it works

Question	Choice
Who sees it?	A screen reader reads it. A sighted user does not see it, until it has the focus (then it shows, with an outline), like the "Skip to main content" link
What does it do?	Scrolls to the top and puts the focus on the first thing that can be reached: the "Skip to main content" link. A plain # link would only scroll, and the focus would stay at the bottom
Address and history	Not changed: the click is cancelled
Where is it?	Once in App.tsx, after the page content, so no page has to add it
Colors	Theme colors only, so it works in light and dark mode

Testing

• Component tests: hidden until focused, scrolls and moves the focus, does not follow the link.
• App test: the link comes after the page on the Search, About and Calculator routes.
• tsc -b, ESLint and all 316 frontend tests pass.
• Checked in a real browser (Chromium) on several pages: the link is 1 by 1 pixels when it does not have the focus, shows with an outline when it has, Enter goes to the top and puts the focus on the skip link, and the address does not change. axe-core finds no violations in light and dark mode with the link focused.

Not in this PR

• A screen reader was not used; that has to be done by a person.
• PR feat(about): add the About page #164 (the About page) first had this link and does not have it any more.
• src/App.tsx and docs/AI_USAGE.md are also edited by other open PRs, so a small conflict is possible. Keep both sides.

🤖 Generated with Claude Code

[xeniag]

Tested both automated and manually, the feature worked. Good job!