feat(ui): add scroll-to-latest button in assistant feed

Track the assistant feed scroll position and footer height, and show a
floating button to jump back to the newest message when scrolled up.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/i18n/locales/en-US.json

@@ -117,6 +117,7 @@
   "conversation": "Chat",
   "newChat": "New chat",
   "conversationHistory": "History",
+  "scrollToLatest": "Scroll to latest",
   "searchConversationPlaceholder": "Search conversations",
   "deleteConversation": "Delete conversation",
   "noConversationHistory": "No history yet",

frontend/src/i18n/locales/zh-CN.json

@@ -117,6 +117,7 @@
   "conversation": "对话",
   "newChat": "新对话",
   "conversationHistory": "历史对话",
+  "scrollToLatest": "回到底部",
   "searchConversationPlaceholder": "请输入搜索关键词",
   "deleteConversation": "删除对话",
   "noConversationHistory": "暂无历史对话",