fix(fields): make FieldCard keyboard-accessible [Phase 2b]

[barach6662001-bit]

Summary

Phase 2b — narrow accessibility fix on FieldCard paying down the debt left over from Phase 2a (#627). Visual contract preserved verbatim.

Why the fallback button-pattern (and not a real link)

A real <a> / react-router <Link> would have been the cleanest semantic answer, but is incompatible with this PR's constraints:

1. Card's as prop only accepts block-level tags; adding 'a' would require touching the Card / Surface API, which is forbidden by the brief.
2. Wrapping Card in <Link> would nest the hover-overlay's real <button> inside an <a> — invalid HTML per the WHATWG spec.
3. The test brief explicitly requires both Enter and Space activation. Native anchor semantics activate on Enter only — that's a button affordance.

The accepted-fallback button-pattern matches the test requirements exactly and stays inside scope.

Changes

• role="button", tabIndex={0}, onKeyDown on the Card surface — activation on Enter and Space (preventDefault on Space stops page scroll).
• aria-label summarises the tile's primary content (name + area + crop when known) so screen readers announce a useful destination instead of button.
• The hover-overlay CTA is now a presentational <span> instead of a real <button> — the parent surface is the activation target, and nesting a real button inside a role="button" element is invalid markup. The overlay container itself is marked aria-hidden because its contents duplicate the card's own aria-label.
• CSS module gains a token-driven :focus-visible ring (outline: 2px solid var(--brand); outline-offset: 2px) and also surfaces the hover-overlay on :focus-visible so keyboard users see the same affordance mouse users do. .card:focus is reset to outline: none so mouse-driven focus stays ring-free, matching the platform-default focus model.

Out of scope (preserved verbatim)

• All visual styling, layout, navigation route and i18n strings.
• Card / Surface API. No new dependencies.
• FieldStatusCard, KpiHeroRow, AppLayout, Sidebar, FieldDetail, OperationDetail.

Tests

__tests__/FieldCard.test.tsx rewritten to 11 cases (was 4):

#	Case
1	primary render — name + area + cadastral + crop
2	cadastral omitted when undefined
3	hover-CTA label is present but is NOT exposed as a button role
4	not-seeded fallback when currentCrop is undefined
5	card has accessible role="button" and full aria-label (with crop)
6	card has accessible aria-label (without crop)
7	card is keyboard-reachable (tabIndex=0)
8	click → navigate
9	Enter → navigate
10	Space → navigate
11	unrelated keys (Tab, Escape, Shift) → no navigate

Validation

• npx vitest run src/design-system src/pages/Fields → 69/69
• npx tsc --noEmit clean
• npx eslint on touched files clean
• npx vite build succeeds (PWA SW generated; pre-existing chunk-size warning unchanged)

Files

File	Change
frontend/src/pages/Fields/components/FieldCard.tsx	role/tabIndex/onKeyDown/aria-label on Card, button→span on overlay CTA, JSDoc updated
frontend/src/pages/Fields/components/FieldCard.module.css	:focus-visible ring, focus-coupled hover overlay, :focus outline reset
frontend/src/pages/Fields/components/__tests__/FieldCard.test.tsx	rewritten — 4 → 11 focused cases

Phase progression

Phase 1a–1f migrated the dashboard row to the Card primitive. Phase 2a (#627) extended that to FieldCard and surfaced the legacy a11y debt (onClick on a non-semantic wrapper). Phase 2b closes that debt without touching the visual contract or the design-system API.