fix(ui): prevent hero flicker when clearing sender input

Only navigate (applyFilters) when a person is actually selected, not
when the sender input is cleared. Combined with showHero checking
data.filters.senderId, the user stays in the search bar view after
clearing — no jump back to the hero.

Refs: #179

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

frontend/src/routes/briefwechsel/+page.svelte

@@ -28,6 +28,8 @@ $effect(() => {
 });
 
 const isSinglePerson = $derived(!!senderId && !receiverId);
+const showHero = $derived(!senderId && !data.filters.senderId);
+
 let showAdvanced = $state(false);
 
 const RECENT_STORAGE_KEY = 'korrespondenz_recent_persons';
@@ -94,8 +96,8 @@ function selectPerson(id: string) {
 }
 </script>
 
-{#if !senderId}
-	<!-- Hero state: centred discovery view -->
+{#if showHero}
+	<!-- Hero state: only on fresh page load with no context -->
 	<div class="mx-auto max-w-7xl px-4 sm:px-6 lg:px-8">
 		<CorrespondenzHero onSelectPerson={selectPerson} recentPersons={recentPersons} />
 	</div>

frontend/src/routes/briefwechsel/CorrespondenzPersonBar.svelte

@@ -76,7 +76,7 @@ function handleSuggestionSelect(id: string) {
 			bind:value={senderId}
 			initialName={initialSenderName}
 			restrictToCorrespondentsOf={receiverId || undefined}
-			onchange={() => onapplyFilters()}
+			onchange={(id) => { if (id) onapplyFilters(); }}
 		/>
 	</div>