feat(search): show spinner in search input while navigation is in-flight

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

frontend/src/routes/+page.svelte

@@ -1,5 +1,6 @@
 <script lang="ts">
 import { goto } from '$app/navigation';
+import { navigating } from '$app/state';
 import { untrack } from 'svelte';
 import { SvelteURLSearchParams } from 'svelte/reactivity';
 import SearchFilterBar from './SearchFilterBar.svelte';
@@ -102,6 +103,7 @@ const showRightColumn = $derived(data.canWrite || (data.incompleteDocs?.length ?
 		bind:tagQ={tagQ}
 		initialSenderName={data.initialValues?.senderName}
 		initialReceiverName={data.initialValues?.receiverName}
+		isLoading={navigating.to !== null}
 		onSearch={handleTextSearch}
 		onfocus={() => (qFocused = true)}
 		onblur={() => (qFocused = false)}