fix(person-mention): respect modified-click and middle-click for new-tab nav

Felix #7: handleMentionClick unconditionally preventDefault'd and goto'd,
breaking ctrl-click / cmd-click / shift-click / alt-click / middle-click —
"open in new tab" is a real workflow for researchers comparing two persons.

Add isPlainPrimaryClick() guard. Modified clicks fall through to the
browser's default anchor handling (the <a href="/persons/{id}"> opens in
the new tab as expected). Plain left-clicks still SPA-navigate via goto().

Three new tests assert ctrl-click, meta-click, and middle-click are not
preventDefault'd.

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

frontend/src/lib/components/TranscriptionReadView.svelte

@@ -143,7 +143,18 @@ function handleMentionLeave(event: Event) {
 	activeCard = null;
 }
 
+/**
+ * Modified clicks (ctrl/meta/shift/alt) and middle-clicks must fall through to
+ * the browser's default anchor behaviour so users can open the person page in
+ * a new tab/window. Felix #7. Only the plain primary-button click navigates
+ * via SPA goto().
+ */
+function isPlainPrimaryClick(event: MouseEvent): boolean {
+	return event.button === 0 && !event.ctrlKey && !event.metaKey && !event.shiftKey && !event.altKey;
+}
+
 async function handleMentionClick(event: MouseEvent) {
+	if (!isPlainPrimaryClick(event)) return;
 	const link = event.target as HTMLAnchorElement;
 	const personId = link.dataset.personId;
 	if (!personId) return;