feat: add insert iframe modal

[wendyyuchensun]

Solves #114 .

Summary

• Adds IframeInputModal — a modal with title and URL fields that inserts an iframe markdown block into the editor
• Extracts isValidUrl from LinkInputModal into src/lib/url.js so both modals share the same validation logic
• Adds the iframe markdown button and markdown syntax insertion support

Demo

Note

1. Why there is no preview panel

The design includes an iframe preview panel inside the modal. I currently remove it because iframe load/error state is not reliably detectable in the browser:

• The onerror event does not fire for the most common failure cases — blocked by X-Frame-Options or Content-Security-Policy: frame-ancestors headers (which most sites
  send)
• The load event fires regardless of whether the content loaded successfully or was blocked
• Cross-origin content prevents any inspection of contentDocument after load

We can discuss if the feature is necessary in future meetings.

2. insert_iframe button svg：設計師還沒確認用哪個 icon，我先暫時先放一個，之後等拿到 svg 檔案再更新