android: downloads on Wi-Fi only

One switch the desktop never needed, off by default: on mobile data every
Save the phone can start — a record, a track, a folder, a file, a
playlist's bulk, the now-playing Save — waits, with the reason on the bar;
streaming is never held back. The walk flips the switch, makes the
connection cellular and sees a download refused.

src/mobile/prefs.svelte.js

@@ -4,7 +4,7 @@
 /* `glass` is the desktop's frosted chrome (a backdrop blur behind the bars).
    Off by default on the phone: tests/mobile/perf.mjs measured it at half the
    frames on a Home scroll and on the player's opening. */
-export const settings = $state({ ambience: true, accent: true, motion: true, glass: false });
+export const settings = $state({ ambience: true, accent: true, motion: true, glass: false, wifiOnly: false });
 
 const KEY = "ss.mobile.prefs";