fix(person-mention): hover card mounts on focusin for keyboard users (WCAG 2.1.1)

Leonie FINDING-01 (Critical) + Elicit E3: only mouseenter triggered the
hover card, so a keyboard user tabbing through transcribed text reached the
anchor but never saw the rich-context preview. For the senior audience
constraint that's a hard regression.

Wire focusin/focusout alongside mouseenter/mouseleave on the delegated
listener. Same handleMentionEnter/Leave run — getBoundingClientRect works
identically on focused elements. focusin/focusout bubble naturally so no
capture phase needed.

Two new tests assert focusin mounts the card and focusout unmounts it.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/components/TranscriptionReadView.svelte

@@ -169,6 +169,10 @@ async function handleMentionClick(event: MouseEvent) {
 // Attach delegated event listeners on each rendered block. Using {@html ...}
 // for the body means we cannot bind events declaratively to the injected
 // anchors, so we hook up listeners via a Svelte action when the wrapper mounts.
+//
+// Keyboard parity (Leonie FINDING-01, WCAG 2.1.1): focusin/focusout mirror
+// mouseenter/mouseleave so users tabbing through transcribed text get the
+// same preview affordance.
 function attachMentionHandlers(node: HTMLElement) {
 	function onEnter(e: Event) {
 		const t = e.target as HTMLElement;
@@ -185,12 +189,17 @@ function attachMentionHandlers(node: HTMLElement) {
 	// mouseenter does not bubble — capture it.
 	node.addEventListener('mouseenter', onEnter, true);
 	node.addEventListener('mouseleave', onLeave, true);
+	// focusin/focusout do bubble — no capture phase needed.
+	node.addEventListener('focusin', onEnter);
+	node.addEventListener('focusout', onLeave);
 	node.addEventListener('click', onClick);
 
 	return {
 		destroy() {
 			node.removeEventListener('mouseenter', onEnter, true);
 			node.removeEventListener('mouseleave', onLeave, true);
+			node.removeEventListener('focusin', onEnter);
+			node.removeEventListener('focusout', onLeave);
 			node.removeEventListener('click', onClick);
 		}
 	};