fix(person-mention): show loading state during debounce + fetch

Leonie #5507 concern 7: on slow networks the popup sat empty for up to
1.5s while the user wondered if anything was happening. Add a loading
flag that flips on as soon as scheduleSearch is asked to query and
back off in the fetch's finally branch. Reuses the existing
comp_typeahead_loading message ("Suche…") so no new i18n keys.

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

frontend/src/lib/components/PersonMentionEditor.svelte

@@ -37,6 +37,7 @@ let query: string | null = $state(null);
 let results: Person[] = $state([]);
 let highlightedIndex = $state(0);
 let mentionStart = $state(0);
+let loading = $state(false);
 
 let textarea: HTMLTextAreaElement | null = null;
 let debounceTimer: ReturnType<typeof setTimeout> | undefined;
@@ -90,8 +91,10 @@ function scheduleSearch(q: string) {
 		// Empty query: keep popup open with last results so the user can browse,
 		// but don't fire a backend call until they actually type something.
 		results = [];
+		loading = false;
 		return;
 	}
+	loading = true;
 	debounceTimer = setTimeout(async () => {
 		try {
 			// SECURITY: relies on the SvelteKit Vite proxy injecting the auth_token
@@ -108,6 +111,8 @@ function scheduleSearch(q: string) {
 			}
 		} catch {
 			results = [];
+		} finally {
+			loading = false;
 		}
 	}, 200);
 }
@@ -140,6 +145,7 @@ function closePopup() {
 	query = null;
 	results = [];
 	highlightedIndex = 0;
+	loading = false;
 	clearTimeout(debounceTimer);
 }
 
@@ -208,7 +214,9 @@ const popupOpen = $derived(query !== null);
 			role="listbox"
 			aria-label={m.person_mention_btn_label()}
 		>
-			{#if results.length === 0}
+			{#if loading}
+				<p class="px-3 py-2 font-sans text-sm text-ink-3">{m.comp_typeahead_loading()}</p>
+			{:else if results.length === 0}
 				<p class="px-3 py-2 font-sans text-sm text-ink-3">{m.person_mention_popup_empty()}</p>
 			{:else}
 				{#each results as person, i (person.id)}