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/HalfSheet.svelte

@@ -18,6 +18,7 @@
     import { lib, albumById, albumsBy, tracksOf, loadLibrary, refreshPlaylists, refreshRecents } from "./lib.svelte.js";
     import { lyr, nudge, searchCandidates, closeCandidates, chooseCandidate, forgetLyrics, pasteLyrics } from "./lyrics.svelte.js";
     import { art, wantArt } from "./art.svelte.js";
+    import { downloadsAllowed } from "./network.svelte.js";
     import { note } from "./notice.svelte.js";
     import { fmtOf, albumFmt, fmtBytes, basename, dirname, longDur, mmss, titleFromName } from "./util.js";
 
@@ -185,6 +186,7 @@
        pipeline — and the whole streamed record the queue holds */
     const streamMates = (track, album) => player.queue.filter((e) => e.t?.stream && e.t.path && e.album?.title === album?.title && e.album?.artist === album?.artist);
     async function saveStreams(entries) {
+        if (!downloadsAllowed()) return;
         const srcs = entries.filter((e) => e.t?.stream && e.t.path).map((e) => ({ username: e.t.stream.username, path: e.t.path, size: e.t.stream.size }));
         if (!srcs.length) return;
         try {
@@ -200,6 +202,7 @@
         if (es.length) playEntries(es, 0, g.folderName).catch(oops);
     }
     async function saveGroup(g) {
+        if (!downloadsAllowed()) return;
         try {
             for (const f of g.files) await api.downloadFile(g.username, f.path, f.size);
             note(`Saving ${g.files.length} file${g.files.length === 1 ? "" : "s"} from ${g.username}`);
@@ -544,7 +547,7 @@
             <div class="sheet-rows">
                 {@render row("play", "Play from here", "the rest of the folder follows", act(() => { const es = groupEntries(g, i); if (es.length) playEntries(es, 0, g.folderName).catch(oops); }))}
                 {#if f}
-                    {@render row("download", "Save this file to library", "as it is, from " + g.username, act(() => api.downloadFile(g.username, f.path, f.size).then(() => note(`Saving “${titleFromName(f.name)}” from ${g.username}`)).catch(oops)))}
+                    {@render row("download", "Save this file to library", "as it is, from " + g.username, act(() => { if (!downloadsAllowed()) return; api.downloadFile(g.username, f.path, f.size).then(() => note(`Saving “${titleFromName(f.name)}” from ${g.username}`)).catch(oops); }))}
                 {/if}
                 <div class="sheet-hair"></div>
                 {@render row("folder-open", "Browse their files", "everything " + g.username + " shares", act(() => push("browse", { username: g.username })))}