a11y: Investigate.tsx CardTag clickable div lacks keyboard support (non-href branch)

[Xore]

Found while doing PR #3182 (design audit P3 structural leg). Out of scope for that PR, tracking separately.

`frontend-next/src/components/Investigate.tsx` around line 339-340: the `CardTag` helper renders an `` when `href` is set, but falls back to a plain `

` with no `role`, `tabIndex`, or `onKeyDown` when there's no href — same WCAG 2.1.1 gap as the payload-workbench run card fixed in #3182, just not named in that task's scope.

Fix: same pattern as #3182 — `role="button"`, `tabIndex={0}`, `aria-label`, Enter/Space `onKeyDown` mirroring the `onClick`, only on the non-href branch.

[Xore]

Fixed via #3197 (merged). CardTag non-href branch now exposes a real keyboard-operable details toggle: visible state caret, aria-expanded/aria-label, card-level closest() guard keeps nested interactives native.