Bluesky identity management should be better

[hellerve]

Currently, only the post snippet does it the right way (using the identity registry). If we want to continue with the identity registry, all snippets should use it. Alternatively, we can switch to a mechanism such as what we use in gt4llm with a keyfile.

Cheers

[hellerve]

Some changes that already happened:

• the link snippet now also offers a login button
• all the login possibilities through the snippets register an identity