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/Mobile.svelte

@@ -47,6 +47,7 @@
     import * as pls from "./playlists.svelte.js";
     import { importer, bindImporter } from "./importer.svelte.js";
     import { bindShare } from "./share.svelte.js";
+    import { downloadsAllowed, cellular } from "./network.svelte.js";
     import * as sources from "../lib/sources.js";
     import { greeting, drag } from "./util.js";
 
@@ -191,7 +192,7 @@
         setTimeout(() => { api.libraryArtPrewarm([360]).catch(() => {}); }, 8000);
         /* the probe handle — the desktop has __soulstream for the same reason:
            tests/mobile/smoke.mjs drives the real app through it */
-        window.__ss = { nav, top, go, push, back, openSheet, closeSheet, openHalf, closeHalf, player, lib, art, slsk, net, resolver, streamSong, streamCatalogAlbum, sources, api, transfers, pls, notice, importer, next, prev, toggle, seekSecs, playQueueAt, patch };
+        window.__ss = { nav, top, go, push, back, openSheet, closeSheet, openHalf, closeHalf, player, lib, art, slsk, net, resolver, streamSong, streamCatalogAlbum, sources, api, transfers, pls, notice, importer, wifi: { downloadsAllowed, cellular }, next, prev, toggle, seekSecs, playQueueAt, patch };
         return () => { if (unlisten) unlisten(); };
     });
 </script>