refactor(sidebar): streamline conversation list with completed filter and expand toggle

Replace flat/grouped view toggle with a show-completed filter dropdown,
add expand/collapse-all control, extract status icon component, and
simplify the sidebar header.

src/i18n/messages/zh-CN.json

@@ -834,8 +834,12 @@
       "statsLabel": "{folders} 个文件夹 · {convos} 个会话",
       "openFolder": "打开文件夹",
       "searchPlaceholder": "搜索会话...",
-      "viewFlat": "平铺视图",
-      "viewGrouped": "按文件夹分组",
+      "showCompleted": "显示已完成会话",
+      "moreOptions": "更多选项",
+      "statusRunningBadge": "运行中",
+      "statusFailedBadge": "失败",
+      "conversationCountUnit": "{count} 条",
+      "emptyFolderHint": "暂无会话",
       "noMatchingConversations": "未找到匹配的会话",
       "removeFolderConfirmTitle": "从工作区移除该文件夹？",
       "removeFolderConfirmDescription": "从工作区移除 \"{name}\"？其相关 Tab 与终端将会关闭。",