feat: social account linking

[luciorubeens]

• Added social account linking so users can map their external username/handle to their address.
• It works through a proof verification similar to keybase.io: users post a message containing their address on the social platform, then submit the proof URL. The backend checks if the proof is valid and updates the status.
• For now it is integrated with X (Twitter) and GitHub. More providers can be added later.
• Updated the backend API to return the associated social handles together with feed, replies, and notifications data when available.

Fixes #508

Dither.Demo.mp4

[jeronimoalbi]

Great work 👍

Something we should consider is to use names like lucio@x or lucio@github instead. Eventually we might want to integrate with Gno.land names registry for example, when the feature is available, and those might eventually be the ones without suffix.

[luciorubeens]

@jeronimoalbi Yes, we're already storing it that way. The only difference is that the Username component will render it as a styled UI badge.

To tag someone, you’ll need to use the user@platform format.

Btw we also need to update the /profile endpoint to support username resolution.