web a11y: notifications markRead is mouse-only (onmouseenter) #62

Flagged during #39 (PR #61): markRead on /notifications fires only on onmouseenter, so keyboard and touch users can never mark a food-sighting notification read. Fix: also mark read on focus (and/or an explicit button with a visible affordance). Acceptance: keyboard-only walkthrough marks a notification read; e2e spec proves it red-first (remove the handler → spec fails); existing mouse behavior unchanged; pnpm --filter web check clean, playwright suite green.