fix(person-mention): close popup on textarea blur

Leonie #5507 concern 1: tabbing away from the editor left the popup
hanging over the next field. Add a 150ms-deferred close on blur — the
delay lets onmousedown on a result fire before the popup unmounts (the
race that the existing onmousedown+e.preventDefault() pattern depends on).

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

frontend/src/lib/components/PersonMentionEditor.svelte

@@ -143,6 +143,14 @@ function closePopup() {
 	clearTimeout(debounceTimer);
 }
 
+function handleBlur() {
+	// Small delay so an option's onmousedown can fire and select before the
+	// popup unmounts. Without this, clicking a result on the way out would
+	// race with blur and lose the selection.
+	setTimeout(() => closePopup(), 150);
+	onblur?.();
+}
+
 function handleKeydown(e: KeyboardEvent) {
 	if (query === null) return;
 
@@ -191,7 +199,7 @@ const popupOpen = $derived(query !== null);
 		oninput={handleInput}
 		onkeydown={handleKeydown}
 		onfocus={onfocus}
-		onblur={onblur}
+		onblur={handleBlur}
 	></textarea>
 
 	{#if popupOpen}