feat: hidden tags (+ skeeted from Witchsky) #9

The atproto client Anisota has a hidden tags option, with #anisota included in every post made through the site. Witchsky should add a dedicated tag for every skeet made through the app, like #witchapp, and allow users to add their own hidden tags to skeets, with the option to save that selection for later use (using a custom lexicon, like app.witchsky.pref.savedtags).

A button to show a hidden tags selection popup could be added next to the Interaction settings and Labels elements in the composer. According to the app.bsky.feed.post schema, there's a limit to 8 additional hashtags per post, so users would be able to select up to 7 of their own.