feat(chat): add collapsible prompt index and polish magazine layout

Add a toggle in the chat header to collapse messages into a compact
prompt index showing user queries with content-type badges. Clicking
a prompt surfaces the corresponding content panel. Also fixes magazine
section extraction (### headers, emoji stripping, author regex) and
improves tile layout with editorial rhythm.

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

packages/app/src/stores/chat.ts

@@ -53,6 +53,7 @@ export const useChatStore = defineStore('chat', () => {
   const panelSide = ref<'left' | 'right'>(savedSide ?? 'left')
 
   const webSearchEnabled = ref(localStorage.getItem('aiui-web-search') !== 'false')
+  const chatCollapsed = ref(localStorage.getItem('aiui-chat-collapsed') === 'true')
 
   // Load chats from server on startup — _loaded gate prevents the watcher
   // from overwriting the file with empty data before the load completes
@@ -81,6 +82,10 @@ export const useChatStore = defineStore('chat', () => {
     localStorage.setItem('aiui-web-search', String(val))
   })
 
+  watch(chatCollapsed, (val) => {
+    localStorage.setItem('aiui-chat-collapsed', String(val))
+  })
+
   watch(
     [conversations, activeConversationId],
     ([conv, active]) => {
@@ -151,6 +156,10 @@ export const useChatStore = defineStore('chat', () => {
     panelSide.value = panelSide.value === 'right' ? 'left' : 'right'
   }
 
+  function toggleChatCollapse() {
+    chatCollapsed.value = !chatCollapsed.value
+  }
+
   function setActiveConversation(id: string) {
     if (conversations.value.has(id)) {
       activeConversationId.value = id
@@ -175,11 +184,13 @@ export const useChatStore = defineStore('chat', () => {
     loaded,
     panelSide,
     webSearchEnabled,
+    chatCollapsed,
     createConversation,
     addMessage,
     appendToLastMessage,
     setMessageWebResults,
     switchSide,
+    toggleChatCollapse,
     setActiveConversation,
     deleteConversation,
   }