Tap to listen, drop marks, bar-timed fades and solar flares

The first visit shows a welcome screen where one tap starts a guest
session; sign-in moves below it. Guests see a "sign in" pill, are
asked once to sign in after five likes or a first follow, and need
two taps to forget themselves.

Drops are found in the waveform and marked on it; "drop" and the d
key jump a bar before the next one. Long and dj-mix fades start on a
bar once the beat is locked. Radio sheets can be shared as /?r=handle.

The audio features now follow the bar, gaps before drops and the
wobble rate, and a new flare pass draws loops round the drop on each
kick, with a setting under visuals.

web/src/api.ts

@@ -139,6 +139,8 @@ export const trackUrl = (t: Track) => `${PL8}/${t.handle}/${t.slug}`;
 /// A link that opens this app on the track.
 export const appTrackUrl = (t: Track) => `${location.origin}/?t=${encodeURIComponent(t.handle)}/${encodeURIComponent(t.slug)}`;
 export const artistUrl = (t: Track) => `${PL8}/${t.handle}`;
+/// A link that opens this app on an artist's radio sheet.
+export const appRadioUrl = (handle: string) => `${location.origin}/?r=${encodeURIComponent(handle)}`;
 // Images come through our server, so pl8list never sees the visitor's address.
 export const imgUrl = (id: string | null) => (id ? `/img/${encodeURIComponent(id)}` : null);
 export const artistImg = (t: Track) => imgUrl(t.artist_img);