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/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>