android: tap-to-sync, the per-track auto-sync switch, the vocal-reduced copy

The editor the desktop drives with a mouse is better under a thumb: the
song plays, the line to time sits large in the middle of the screen with
the one behind and the one ahead faint around it, and a tap anywhere
stamps it. One layer the back gesture peels, and a set left mid-tap is
offered back by the row that opened it; Save writes the track's own
document, lines never tapped left out of it and the button saying how
many are kept. The lyrics sheet also gains the per-track auto-sync switch
and the desktop's reduced-vocals copy — the same title, artist and
duration, so the words keep following it, and one row puts the original
back.

src/mobile/Mobile.svelte

@@ -16,6 +16,7 @@
     import Sheet from "./Sheet.svelte";
     import HalfSheet from "./HalfSheet.svelte";
     import Viewer from "./Viewer.svelte";
+    import SyncRoom from "./SyncRoom.svelte";
     import Home from "./screens/Home.svelte";
     import Search from "./screens/Search.svelte";
     import Library from "./screens/Library.svelte";
@@ -306,6 +307,7 @@
     <HalfSheet />
     <Sheet />
     <Viewer />
+    <SyncRoom />
 
     {#if player.error || lib.error}
         <div class="toast" onclick={() => { player.error = ""; lib.error = ""; }} role="status">{player.error || lib.error}</div>