Allow notifications to link directly to casts

[vrypan]

Overview

Extend targetUrl in the notification payload to allow fc://cast/<fid>/<hash> as valid values. This will allow notifications to link to casts directly (like Farcaster-app notifications do, for example notifications about likes and recasts.)

Use cases

Miniapp Updates, news, instructions

Many miniapps that are not designed to show arbitrary content (for example a token swap, or a game miniapp) do not have a way to easily share updates. They either have to update their code and their design to show arbitrary content, or use the unnatural redirect method, where the user clicks on the notification, opens the miniapp, and is redirected to a cast.

Casts are the native content unit of Farcaster, and it would be a great UX and DX upgrade if the project could just post their news or warnings, or new feature descriptions as casts and have notifications link directly to them.

Cast-centric miniapps

Miniapps like Buoy, notify users about casts that meet specific criteria. Right now, the only possible flow is to click the notification, open the miniapp (and close an other miniapp if you happen to have one open) and get redirected to the cast you wanted to see in the first place.

This flow is very bad UX, because if forces the user to close the current miniapp (and lose state), it is slow, and feels unnatural.

Being able to link directly to a cast would solve all these problems.

Extension: Miniapp feed

A Farcaster client could offer a "miniapp feed" populated by the casts included in notifications. I.e. a custom, first class feed (like Following and Home) generated by the miniapps a user follows.

This would open the door to custom algorithms, from "simple" ones like "all mentions of X" by Buoy, to more advanced ones, operated by bots that search and optimize a custom feed based on user preferences, behavior, transactions, location, language, trending, trending in my current location, etc. The possibilities are endless, and no other social network offers this.

And the great thing is that this feed can be the result of multiple "algorithms".