#3767 CVE pages: horizontally scrolling code blocks are not keyboard-reachable

The pre.cve-code blocks on /cve/dirtyfrag and /cve/mini-shai-hulud (the sample-output pre elements in the routes and the one in islands/PrismCodeBlock.tsx) overflow horizontally at narrow widths but have no tabIndex, accessible name or focus ring, so a keyboard-only user cannot scroll to the clipped part of a command or results table (WCAG 2.1.1 Keyboard). Most visible on the dirtyfrag Mitigation Results commands table at 375px. Fix: give each scrollable pre tabIndex 0, an aria-label naming what it holds, and focus:ring-2. Raised by the ux-review during verification of swamp-club #3761; pre-existing, not introduced there.