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

@@ -92,29 +92,31 @@ const enrichedDocuments = $derived(
 			</div>
 
 			<!-- Swap button — icon-only on desktop, full-width with text on mobile -->
-			<div class="flex items-end">
-				<button
-					data-testid="conv-swap-btn"
-					onclick={swapPersons}
-					class="flex w-full items-center justify-center gap-2 border border-brand-sand px-3 py-2.5 text-xs font-bold tracking-widest text-brand-navy uppercase transition-colors hover:bg-brand-navy hover:text-white md:w-auto"
-					title={m.conv_swap_btn()}
-				>
-					<svg
-						class="h-4 w-4 flex-shrink-0 md:rotate-90"
-						fill="none"
-						stroke="currentColor"
-						viewBox="0 0 24 24"
+			{#if senderId && receiverId}
+				<div class="flex items-end">
+					<button
+						data-testid="conv-swap-btn"
+						onclick={swapPersons}
+						class="flex w-full items-center justify-center gap-2 border border-brand-sand px-3 py-2.5 text-xs font-bold tracking-widest text-brand-navy uppercase transition-colors hover:bg-brand-navy hover:text-white md:w-auto"
+						title={m.conv_swap_btn()}
 					>
-						<path
-							stroke-linecap="round"
-							stroke-linejoin="round"
-							stroke-width="2"
-							d="M7 16V4m0 0L3 8m4-4l4 4M17 8v12m0 0l4-4m-4 4l-4-4"
-						></path>
-					</svg>
-					<span class="md:hidden">{m.conv_swap_btn()}</span>
-				</button>
-			</div>
+						<svg
+							class="h-4 w-4 flex-shrink-0 md:rotate-90"
+							fill="none"
+							stroke="currentColor"
+							viewBox="0 0 24 24"
+						>
+							<path
+								stroke-linecap="round"
+								stroke-linejoin="round"
+								stroke-width="2"
+								d="M7 16V4m0 0L3 8m4-4l4 4M17 8v12m0 0l4-4m-4 4l-4-4"
+							></path>
+						</svg>
+						<span class="md:hidden">{m.conv_swap_btn()}</span>
+					</button>
+				</div>
+			{/if}
 
 			<!-- Receiver -->
 			<div