fix(transcription): add noreferrer to mention dropdown create-new link

For issue #380 (Nora CWE-116). The "Neue Person anlegen" link opens in
a new tab and was missing `noreferrer` — the new tab could read
window.opener and the referrer leaked the transcription URL. Same-origin
risk is low but the omission was unintentional.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/shared/discussion/MentionDropdown.svelte

@@ -188,7 +188,7 @@ function selectItem(item: Person) {
 		<a
 			href="/persons/new"
 			target="_blank"
-			rel="noopener"
+			rel="noopener noreferrer"
 			class="flex min-h-[44px] items-center gap-2 border-t border-line px-3 py-2.5 font-sans text-sm font-medium text-brand-navy hover:bg-canvas focus:bg-canvas focus:outline-none"
 			onmousedown={(e) => e.preventDefault()}
 		>