feat: display conversation subject in list view

Show the subject line between the contact name and inbox name
in the conversation list. This helps agents triage tickets
without having to open each conversation and navigate to the
Information panel.

Closes #239

frontend/src/features/conversation/list/ConversationListItem.vue

@@ -42,6 +42,11 @@
               </span>
             </div>
 
+            <!-- Subject -->
+            <p v-if="conversation.subject" class="text-xs font-medium text-muted-foreground truncate">
+              {{ conversation.subject }}
+            </p>
+
             <!-- Inbox name -->
             <p class="text-xs text-gray-400 flex items-center gap-1.5">
               <Mail class="w-3.5 h-3.5 text-gray-400/80" />