refactor(multiboards): remove time filter system end-to-end

Backend retention now keeps only recent posts, making the time filter UI
and newerThan filtering obsolete. Deleted use-time-filter hook, canonical
multiboard routes only, removed footer suggestions, dropped i18n keys.

public/translations/zh/default.json

@@ -135,7 +135,6 @@
   "sort_by": "排序方式",
   "subplebbit_offline_info": "此看板可能处于离线状态，发布可能会失败",
   "posts_last_synced_info": "帖子最后同步于 {{time}}，此看板可能处于离线状态，发布可能会失败。",
-  "newer_than": "新于",
   "no_threads": "没有帖子",
   "hidden": "隐藏",
   "save_options": "保存选项",
@@ -181,8 +180,6 @@
   "no_board_selected_warning": "请在发布之前选择一个板块。",
   "invalid_url_alert": "提供的链接不是有效的 URL。",
   "empty_comment_alert": "无法发布空评论。",
-  "more_threads_last_week": "{{count}} 主题自 {{currentTimeFilterName}} — [<1>显示更多来自上周的主题</1>]",
-  "more_threads_last_month": "{{count}} 主题自 {{currentTimeFilterName}} — [<1>显示更多上个月的主题</1>]",
   "newer_threads_available": "有新的帖子可用 — [<1>刷新</1>]",
   "stored_locally": "本地存储（位于 {{location}} 数据中），不会在设备间同步",
   "choose_one": "选择一个：",
@@ -191,7 +188,6 @@
   "reset_changes": "重置更改",
   "crypto_address_yours": "加密地址属于此帐户。",
   "delete_wallet": "删除钱包",
-  "last_bumped": "最后提升",
   "fit_expanded_images_to_screen": "适应扩展图像到屏幕",
   "fit_expanded_images_to_screen_tip": "将扩展图像限制为浏览器的宽度和高度",
   "choose_file": "选择文件",
@@ -222,7 +218,6 @@
   "filter_and_highlights_help": "过滤器和高亮帮助",
   "filter_and_highlights": "筛选器与高亮",
   "boards_you_moderate": "您管理的板块",
-  "more_threads_last_year": "{{count}} 主题自 {{currentTimeFilterName}} — [<1>显示更多去年的主题</1>]",
   "moderator_of_this_board": "此用户是此版块的版主",
   "administrator_of_this_board": "此用户是此版块的管理员",
   "copy_content_id": "复制内容ID",