feat(conversations): add swap button (#32)

Adds a button between the two person typeaheads that swaps sender and
receiver, then reloads the conversation view.

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

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

@@ -37,6 +37,13 @@ function toggleSort() {
 	sortDir = sortDir === 'DESC' ? 'ASC' : 'DESC';
 	applyFilters();
 }
+
+function swapPersons() {
+	const tmp = senderId;
+	senderId = receiverId;
+	receiverId = tmp;
+	applyFilters();
+}
 </script>
 
 <div class="mx-auto max-w-5xl px-4 py-10">
@@ -80,6 +87,27 @@ function toggleSort() {
 			</div>
 		</div>
 
+		{#if senderId && receiverId}
+			<div class="mb-4 flex justify-center">
+				<button
+					data-testid="conv-swap-btn"
+					onclick={swapPersons}
+					class="flex items-center gap-2 rounded-sm border border-brand-sand px-4 py-2 text-xs font-bold tracking-widest text-brand-navy uppercase transition-colors hover:bg-brand-navy hover:text-white"
+					title={m.conv_swap_btn()}
+				>
+					<svg class="h-4 w-4" 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>
+					{m.conv_swap_btn()}
+				</button>
+			</div>
+		{/if}
+
 		<div class="relative z-10 grid grid-cols-1 items-end gap-6 md:grid-cols-3">
 			<!-- Date From -->
 			<div>