Support Signing via a browser plugin

[NfNitLoop]

One thing I liked about the Nostr ecosystem is that they had browser plugins that would allow you to sign things without having to constantly input your private key. We should do that for Diskuto.

Right now, there's a "signing tool" that does this in the browser. While that's somewhat secure since I know the implementation, it's difficult to trust that as a user.

Write a plugin that:

• Preferably works across Chrome/Firefox/Safari without needing different codebases.

• DOES NOT require read access to the web page (i.e.: all web pages) until a user clicks on the extension button.

• Supports multiple identities

• Saves private keys for identities, preferably with an attached "name" so it's easy to distinguish them.

• When the user clicks the plugin icon in their browser, it should:
  * Look for a signing request on the page. (simplest: hidden <input> element.)
  * Prompts the user to confirm they want to sign as a given user.
  * Places the signature into the signature field on the page.