fix(gateway): announce declined 'all' scope in /sessions and /resume listings

A non-admin '/sessions all' or '/resume --all' silently downgraded to
chat-scoped listing with zero feedback, which reads as 'my session
vanished' (community Telegram report). Both surfaces now append a notice
that cross-chat listing requires a configured admin. Follows up the
salvaged current-session '(current)' marker (PR #68556, fixes #68547):
sibling tests updated to pin the new contract, new i18n key
gateway.resume.all_requires_admin added to all 17 locales, docs updated.

locales/zh.yaml

@@ -269,6 +269,7 @@ Use `/title My Session` to name the current room session, `/resume --all` to lis
 Future messages in this room will use that transcript until `/reset` or another `/resume`.{msg_part}"
     blocked_not_owner:     "⚠️ /resume blocked: '**{name}**' belongs to a different user or chat. You can only resume sessions from this chat."
     no_named_sessions:     "未找到已命名的会话。\n使用 `/title 我的会话` 为当前会话命名，然后用 `/resume 我的会话` 返回。"
+    all_requires_admin: "_注意：`--all`（跨聊天列表）需要已配置的管理员；仅显示本聊天的会话。_"
     list_header:           "📋 **已命名会话**\n"
     list_item:             "• **{title}**{preview_part}"
     list_item_numbered:    "{index}. **{title}**{preview_part}"