Added a setup guidance page (setup.html) that opens when native messaging fails (e.g., desktop app not installed or not running). Previously, the error was only logged to console.error with no user feedback.
The setup page guides the user to install the desktop app from the Microsoft Store and enable browser support. It includes a Retry button to re-test connectivity.
Rate limiting prevents the setup page from opening repeatedly on rapid clicks (10s cooldown).
Added i18n strings for the setup page (English + Simplified Chinese).

Note: This extension requires a native messaging host registered by the Easydict desktop app. Without it, clicking the context menu item will open the setup page — this is the intended behavior for first-time users who haven't completed setup. The extension does not auto-open any pages on install; the setup page only appears in response to a user-initiated action (context menu click) that fails.