GHSA-x34j-47hf-4xg7: TinaCMS admin preview iframe loads an attacker-controlled origin from the URL fragment

The TinaCMS admin builds its preview from the /~/* hash-router splat without checking that the value stays same-origin. A fragment with a doubled slash (#/~//attacker.example/p) becomes the protocol-relative URL //attacker.example/p, so the admin frames an external site. That same unvalidated string derives expectedOrigin, the only trust anchor for the admin↔preview postMessage channel, so the attacker's frame is treated as trusted: it can submit any GraphQL operation, which the admin executes with the signed-in editor's token and posts back to the attacker's origin.