fix(files): hide desktop-only reveal action and label download in browser clients

Reveal-in-file-manager was always offered whenever the server exposed
revealPath, including in a plain browser tab where there is no local
file manager to reveal into. Gate it behind a new isBrowserClientRuntime
check (web platform, no Electron shell) and relabel the save action to
"Download" for that case, since it triggers a browser-style file
download rather than an in-place save.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

packages/ui/src/lib/i18n/messages/ja.ts

@@ -1221,6 +1221,7 @@ export const dict: Record<I18nKey, string> = {
   'sidebarFilesTree.menu.rename': '名前の変更',
   'sidebarFilesTree.menu.copyPath': 'パスをコピー',
   'sidebarFilesTree.menu.save': '保存',
+  'sidebarFilesTree.menu.download': 'ダウンロード',
   'sidebarFilesTree.menu.newFile': '新しいファイル',
   'sidebarFilesTree.menu.newFolder': '新しいフォルダ',
   'sidebarFilesTree.menu.delete': '削除',