Make import candidate selection a native radiogroup

[adamXbot]

Summary

The iTunes match candidates in the onboarding wizard's SearchResultBlock were bare clickable <div>s — no role, no tabIndex, no keyboard path, invisible to assistive tech. This converts them to <button role="radio"> rows inside a labelled role="radiogroup", reusing the wizard's existing roving-radiogroup pattern (lib/use-roving-radiogroup.ts, the same one the method and AI-provider cards use):

• Tab enters the group on the chosen row (first row when nothing is chosen, via rovingTabIndex); arrows move with selection-follows-focus; Home/End and RTL flipping come with the shared hook.
• Click/Space on the already-chosen row still clears it — existing toggle behaviour preserved; the hook's checked guard keeps arrow passes from clearing rows in transit.
• The group's aria-label carries the query (new i18n key onboard.search_block.candidates_group_aria, en + zh).
• The decorative checkmark span is aria-hidden and the candidate icon's alt is now empty, so the row's accessible name is the candidate name/developer text rather than the name read twice.
• .candidate-row CSS neutralises the UA button chrome — visual layout is unchanged.

Test changes

The fixme-gated candidate-selection spec in tests/e2e/onboarding-keyboard.spec.ts — committed with the a11y gate (#134) as this fix's acceptance test — is now live: it walks the wizard keyboard-only, steps into the radiogroup with Shift+Tab, and selects the non-default Apple candidate with ArrowDown, asserting focus, aria-checked, and the chosen state.

Verification

• Full Playwright suite: 34 passed / 0 skipped (the suite's one standing skip was this spec) — includes the axe scan of the match step with the new markup and all click-based candidate flows in onboard-import.spec.ts.
• 438 unit tests, 0 failures; pnpm typecheck, pnpm lint, pnpm lint:i18n (4,760 keys at parity) all clean.

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