fix(web): keyboard navigation and layout fixes in the instrument list and sidebar

[thomasbeaudry]

Splits #1434 into three PRs. This is PR 1 of 3 and merges first.

Merge order

Order	PR	Branch	Base
1st	this PR — UX fixes	split/ux	main
2nd	#1439 — mailer	split/mailer	main
3rd	#1438 — Spanish + active languages	split/language	split/mailer

This PR is fully independent of the other two: it adds zero new t() calls, so nothing in it changes when Spanish lands. It goes first because it's the smallest and carries no cross-PR constraints. Verified: merging all three in this order produces zero conflicts.

What's here

• Instrument showcase is keyboard navigable — arrow keys move a highlight through the filtered results, Enter selects, and the highlighted card scrolls into view. Focus moves to the search input on entering the accessible instruments page. Individual cards lose their own tab stop so the list is traversed as one control rather than one stop per card.
• Sidebar nav scrolls independently once it outgrows the viewport, instead of pushing the session card and footer off screen.
• Nav groups collapse when navigation leaves them, so at most one group is open at a time. Note this reverses a deliberate behaviour documented in the code on main (a group the user opened stayed open as they navigated away) — worth a second look if that was intentional.
• Audit Logs moves above Branding in the admin nav; QR codes render at a smaller scale.

Dropped from #1434

The whitespace realignment of the InstrumentRepo model in schema.prisma is not carried over. It is incidental editor churn, and prisma format on current main produces a different, larger realignment (it also touches InstrumentRecord), so replaying it would have added unrelated noise rather than a formatting fix.

Verification

tsc, eslint, and prettier --check clean for web; pnpm test 307 passed / 1 skipped.

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