[Feature]: Embed code — generate iframe snippet to embed a position on any website

[BilgeGates]

What problem does this solve?

Chess bloggers, teachers, and forum users who want to display a position on their own website currently have no way to do so from ChessVision. They must export an image and host it separately — losing interactivity and breaking if the image moves.

Proposed solution

Add an Embed action that generates a ready-to-copy <iframe> snippet for the current position.

Generated output:

<iframe
  src="https://chessvision.org/embed?fen=rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"
  width="400"
  height="400"
  frameborder="0"
  allowfullscreen>
</iframe>

Scope for this release:

• /embed route that renders the board in minimal UI (no navbar, no panels)
• Size selector before copying (small 320px / medium 480px / large 640px)
• Board theme and piece set reflected in the embed URL as query params
• One-click copy to clipboard
• Works from the main board editor

Out of scope:

• Interactive piece movement inside the embed
• Move list playback
• Custom branding/colors via embed params

Who needs this

Chess bloggers, online teachers, Wikipedia editors, Reddit/forum users, and anyone publishing chess content on the web. Lichess offers this feature and it is one of their most-used sharing tools.

Before submitting

• I searched existing requests and this is not a duplicate.