fix(ui): keep file viewer visible while installing

The detail view already exposes View Files while a game is downloaded,
installed, or actively downloading. During the download-to-install handoff,
the backend can report Installing before the local downloaded/installed flags
have settled in the next snapshot, which briefly hid the file viewer button.

Treat Installing as another state where the game root should remain reachable.
This keeps the detail view stable during the handoff without changing backend
file-opening behavior.

Test Plan:
- just frontend-test

crates/lanspread-tauri-deno-ts/src/components/modals/GameDetailModal.tsx

@@ -55,7 +55,8 @@ export const GameDetailModal = ({
         && !isInProgress(game.install_status);
     const canViewFiles = game.downloaded
         || game.installed
-        || game.install_status === InstallStatus.Downloading;
+        || game.install_status === InstallStatus.Downloading
+        || game.install_status === InstallStatus.Installing;
     return (
         <Modal onClose={onClose}>
             <button className="modal-close" type="button" onClick={onClose} aria-label="Close">