[Bug]: Embedding functionality broken for new /f/ and /p/ share links
### What happened?

Before tldraw.com added login functionality, we could successfully create embeds by either pasting tldraw links directly or using the embed menu. This no longer works with new share links.

Our team has a long-used "master whiteboard" that aggregates various sketches and discussions. Recently we discovered that simply copying new links (starting with /f/ or /p/) can't create embeds like before, though modifying them to /r/ paths still works.

From briefly checking the source code, this likely relates to the [regex constraint](https://github.com/tldraw/tldraw/blob/60a92b7ae37774a3e8bb5403469fb098b03244bf/packages/tlschema/src/shapes/TLEmbedShape.ts#L8)

### How can we reproduce the bug?

![Image](https://github.com/user-attachments/assets/7b65da86-156d-4b68-a5d0-023266c69466)

Original new link: https://tldraw.com/f/abc123

Modified legacy format: https://tldraw.com/r/abc123
→ Same content loads and embeds successfully

![Image](https://github.com/user-attachments/assets/5d92be26-9185-4912-8d9c-bd0c264eb515)

### What browsers are you seeing the problem on?

Chrome

### Where did this happen?

The tldraw.com website

### Contact Details

_No response_

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