feat(memoir+conversation): 章节/故事最小可读字数；会话 hasUserMessage 与 UI 优化

- 后端：300 字门槛统一物化、hydrate、列表/PDF/详情；过短章节对读者隐藏
- 对话：首包前打字动画、大字模式排版、朗读/TTS 交互与布局稳定
- 首页：复用无用户消息会话；空列表「继续对话」与文案 i18n
- 章节阅读：标题进正文、封面与去重标题；阅读 Markdown 字号上调

app-expo/src/i18n/locales/en/conversation.json

@@ -11,6 +11,7 @@
   "deleteConversation": "Delete Conversation",
   "addMore": "More",
   "agentName": "Life Echo",
+  "assistantReplying": "Replying…",
   "cancelRecording": "Cancel recording",
   "chatTitle": "Conversation",
   "connectionConnected": "Connected",
@@ -18,6 +19,8 @@
   "connectionDisconnected": "Disconnected",
   "emptyGreetingSubtitle": "Chat with your companion and record your stories.",
   "greetingTitle": "Say Hello",
+  "resumeChatTitle": "Continue chatting",
+  "resumeChatSubtitle": "Open your latest conversation to keep talking.",
   "inputPlaceholder": "Type a message...",
   "inputPlaceholderVoice": "Type here or hold the mic to speak...",
   "me": "Me",