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/en.json

@@ -834,8 +834,12 @@
       "statsLabel": "{folders} folders · {convos} conversations",
       "openFolder": "Open Folder",
       "searchPlaceholder": "Search conversations...",
-      "viewFlat": "Flat view",
-      "viewGrouped": "Grouped by folder",
+      "showCompleted": "Show completed conversations",
+      "moreOptions": "More options",
+      "statusRunningBadge": "Running",
+      "statusFailedBadge": "Failed",
+      "conversationCountUnit": "{count, plural, one {# conversation} other {# conversations}}",
+      "emptyFolderHint": "No conversations",
       "noMatchingConversations": "No matching conversations",
       "removeFolderConfirmTitle": "Remove folder from workspace?",
       "removeFolderConfirmDescription": "Remove \"{name}\" from the workspace? Its tabs and terminals will close.",