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-TW.json

@@ -834,8 +834,12 @@
       "statsLabel": "{folders} 個資料夾 · {convos} 個對話",
       "openFolder": "開啟資料夾",
       "searchPlaceholder": "搜尋對話...",
-      "viewFlat": "平鋪視圖",
-      "viewGrouped": "依資料夾分組",
+      "showCompleted": "顯示已完成對話",
+      "moreOptions": "更多選項",
+      "statusRunningBadge": "運行中",
+      "statusFailedBadge": "失敗",
+      "conversationCountUnit": "{count} 條",
+      "emptyFolderHint": "暫無對話",
       "noMatchingConversations": "找不到符合的對話",
       "removeFolderConfirmTitle": "從工作區移除此資料夾？",
       "removeFolderConfirmDescription": "從工作區移除 \"{name}\"？相關分頁與終端機將會關閉。",