chore: remove About link from header and language switcher from about pages

- Remove About link and sponsor notice icon from chat panel header
- Remove language switcher (English | 中文 | 日本語) from all about pages
- Fix About link in settings dialog to use current language
- Remove unused sponsorTooltip translation key from all dictionaries

lib/i18n/dictionaries/en.json

@@ -18,8 +18,7 @@
         "settings": "Settings",
         "hidePanel": "Hide chat panel (Ctrl+B)",
         "showPanel": "Show chat panel (Ctrl+B)",
-        "aiChat": "AI Chat",
-        "sponsorTooltip": "Sponsored by ByteDance Doubao K2-thinking. See About page for details."
+        "aiChat": "AI Chat"
     },
     "providers": {
         "useServerDefault": "Use Server Default",

lib/i18n/dictionaries/ja.json

@@ -18,8 +18,7 @@
         "settings": "設定",
         "hidePanel": "チャットパネルを非表示 (Ctrl+B)",
         "showPanel": "チャットパネルを表示 (Ctrl+B)",
-        "aiChat": "AI チャット",
-        "sponsorTooltip": "ByteDance Doubao K2-thinking によるスポンサー。詳細は概要ページをご覧ください。"
+        "aiChat": "AI チャット"
     },
     "providers": {
         "useServerDefault": "サーバーデフォルトを使用",

lib/i18n/dictionaries/zh.json

@@ -18,8 +18,7 @@
         "settings": "设置",
         "hidePanel": "隐藏聊天面板 (Ctrl+B)",
         "showPanel": "显示聊天面板 (Ctrl+B)",
-        "aiChat": "AI 聊天",
-        "sponsorTooltip": "由字节跳动豆包 K2-thinking 赞助。详情请参阅关于页面。"
+        "aiChat": "AI 聊天"
     },
     "providers": {
         "useServerDefault": "使用服务器默认值",