Feature: Implement a URI scheme for Diskuto #133

See: https://www.tbray.org/ongoing/When/202x/2025/04/16/Decentralized-Schemes

Currently, Diskuto "decentralizes" its links in a couple ways:

• The recommended way to link to another post is with a relative link of the format /u/:uid/i/:sig/. That lets links "just work" as long as other web interfaces use the same layout.
• Item data just stores the binary-encoded uid/sig, and makes no reference to a particular server.

But yeah, it would be good to bite the bullet and define a URI scheme. Then we could link to something like diskuto:/post/:uid/:sig/, and clients could normalize that to whatever is appropriate for their view.