feat(sidechat): rename feature label to 侧边对话(beta) / Side Chat (beta) (#307)

The sidechat feature is still experimental; rename its display label so
users see it is beta. The label reads from the single sideChat locale
key, which drives both the "+" new-tab menu entry and the Side card
settings card title, so both update with this one change. Update the
e2e mount sweep expected built-in label and the README feature bullets.

src/client/locales.ts

@@ -247,7 +247,7 @@ export const zh = {
   subagentDiagUnsupported: '不支持的条目',
   subagentDiagUnavailable: '不可用',
   subagentThinking: '思考中…',
-  sideChat: '侧边对话',
+  sideChat: '侧边对话(beta)',
   sideChatNew: '新建对话',
   sideChatUntitled: '新对话',
   sideChatEmpty: '暂无侧边对话',
@@ -553,7 +553,7 @@ export const en: Record<keyof typeof zh, string> = {
   subagentDiagUnsupported: 'Unsupported',
   subagentDiagUnavailable: 'Unavailable',
   subagentThinking: 'Thinking…',
-  sideChat: 'Side Chat',
+  sideChat: 'Side Chat (beta)',
   sideChatNew: 'New thread',
   sideChatUntitled: 'New thread',
   sideChatEmpty: 'No side conversations',