fix(conversations): restore {#if} guard on swap button

The guard was lost when the button was moved into the grid between the
two person inputs. Without it the button rendered even when no persons
were selected, breaking the UX and the E2E assertion.

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

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

@@ -92,6 +92,7 @@ const enrichedDocuments = $derived(
 			</div>
 
 			<!-- Swap button — icon-only on desktop, full-width with text on mobile -->
+			{#if senderId && receiverId}
 				<div class="flex items-end">
 					<button
 						data-testid="conv-swap-btn"
@@ -115,6 +116,7 @@ const enrichedDocuments = $derived(
 						<span class="md:hidden">{m.conv_swap_btn()}</span>
 					</button>
 				</div>
+			{/if}
 
 			<!-- Receiver -->
 			<div