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/screens/You.svelte

@@ -242,6 +242,11 @@
 
 <div class="sheet-sec">Downloads</div>
 <div class="group">
+    <button class="row" onclick={() => setSetting("wifiOnly", !settings.wifiOnly)}>
+        <span class="lead"><Icon name="globe" size={20} /></span>
+        <span class="b"><span class="t">Downloads on Wi-Fi only</span><span class="m">on mobile data, Save waits; streaming is never held back</span></span>
+        <span class="sw" class:on={settings.wifiOnly}><span></span></span>
+    </button>
     <button class="row" onclick={layoutPrompt}>
         <span class="lead"><Icon name="folder-open" size={20} /></span>
         <span class="b"><span class="t">Album folder format</span><span class="m mono">{cfg?.library_layout || "{artist}\\{album}"}</span></span>