feat(gymnasts): add the gymnast page

[ronjalh]

Summary

The Gymnast page: the page for one gymnast, reached from a card on "Your gymnasts". It shows one apparatus at a time, with the routines and their D-scores, and the list of elements of the gymnast on that apparatus.

What is in it

• Route /gymnasts/:gymnastId behind the login check. A visitor who is not logged in is sent to the login page. An unknown or foreign id shows a clear "not found".
• Apparatus tabs: the shadcn/ui tabs in the "line" look (added with the shadcn CLI, on the Base UI tabs, so arrow keys, Home and End work). Each tab has only the name of the apparatus.
• The routines of the chosen apparatus, highest D-score first. The highest one is marked with text, not by color alone.
• The list of elements with a remove button for each element. The accessible name says which element. A confirmation dialog comes first, and a role="status" message tells what happened.
• Loading, error, empty and not-found states.
• Each card on "Your gymnasts" links to the page: the name is a link inside the heading, and the link covers the whole card. The delete button stays above it.
• The "are you sure?" dialog of DeleteGymnastButton is now a shared ConfirmDialog, used by both the delete button and the removal of an element.

Choices

• One query asks for the gymnast with all routines and elements (the API limits them). The page groups them by apparatus itself, so changing apparatus costs no request.
• No new colors: the page reuses the existing tokens and components (PageTitle, Button, DifficultyBadge).
• Two small dark-mode rules in global.css make the value under the difficulty badge readable on the dark card. The badge itself is not changed.

Testing

• npm run lint, tsc -b and npm run build are clean.
• npm run test:run: 52 files, 633 tests pass. New tests cover the grouping logic, the page on the real Apollo Client with a stub for fetch, the confirmation dialog, the link from the cards, and the redirect to the login page.
• Checked by hand with axe-core in light and dark mode, with the keyboard, and at 320 px (see docs/AI_USAGE.md). A screen reader was not used.

Open points

• Not in this PR: adding a routine (Create functionality for adding a routine to a gymnast #152), "add to gymnast" on the element page, renaming the gymnast and deleting a routine.
• Draft until the team has looked at the design of the tabs.

Part of #149

🤖 Generated with Claude Code

[xeniag]

The build runs, passed all test including axe-core accessibility tests. Looked fine when I ran it locally too, the functionality for adding elements to a gymnast works well. Light and dark mode is ok also.