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/HalfSheet.svelte

@@ -16,7 +16,7 @@
     import { DOORS, start as startImport } from "./importer.svelte.js";
     import { slsk, refreshRooms, disconnect, openThread, addBuddy, removeBuddy, leaveRoom } from "./slsk.svelte.js";
     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 { lyr, nudge, searchCandidates, closeCandidates, chooseCandidate, forgetLyrics, pasteLyrics, setAutoSync, openTapSync, tapProgress, reduceVocals, backToOriginal } from "./lyrics.svelte.js";
     import { art, wantArt } from "./art.svelte.js";
     import { metaOf, display, realName, setMeta, clearMeta } from "./contacts.svelte.js";
     import { downloadsAllowed } from "./network.svelte.js";
@@ -69,6 +69,11 @@
         }
         return null;
     });
+    /* the lyrics sheet: a tap-sync set left mid-tap for the playing track,
+       and whether a vocal-reduced copy is what is playing */
+    const tapLeft = $derived(kind === "lyrics" ? tapProgress() : null);
+    const canReduce = $derived(!!player.track?.path && !player.track?.stream);
+    const onReduced = $derived(!!lyr.reduced && player.track?.path === lyr.reduced.to);
     const onThisAlbum = $derived((id) => top().n === "album" && top().c.id === id);
     const after = $derived(Math.max(0, player.queue.length - player.qi - 1));
 
@@ -281,6 +286,14 @@
     </button>
 {/snippet}
 
+{#snippet swrow(icon, t, m, on, fn)}
+    <button class="row" onclick={fn}>
+        <span class="lead"><Icon name={icon} size={20} /></span>
+        <span class="b"><span class="t">{t}</span>{#if m}<span class="m">{m}</span>{/if}</span>
+        <span class="sw" class:on><span></span></span>
+    </button>
+{/snippet}
+
 <div class="halfsheet" class:up={!!nav.half}>
     <div class="grab"></div>
     <div class="hs-body">
@@ -621,10 +634,17 @@
                     {#if lyr.lines.length}
                         {@render row("skip-back", "Lyrics earlier", "half a second, kept for this track", () => nudge(-500))}
                         {@render row("skip-forward", "Lyrics later", "half a second", () => nudge(500))}
+                        {@render swrow("history", "Auto-sync this track", lyr.autoOffset ? `correcting by ${lyr.autoOffset > 0 ? "+" : ""}${(lyr.autoOffset / 1000).toFixed(1)} s` : "let the app correct this track's timing", !lyr.autoSyncOff, () => { void setAutoSync(lyr.autoSyncOff); })}
                         <div class="sheet-hair"></div>
                     {/if}
                     {@render row("search", "Wrong lyrics? Search again", "LRCLIB, by this track's name", () => { void searchCandidates(); })}
                     {@render row("file-text", "Paste lyrics…", "plain text, or LRC with [mm:ss] timestamps", () => openHalf("prompt", { title: "Paste lyrics", placeholder: "[00:12.50] The first line…", ok: "Save", multiline: true, onsubmit: (v) => { void pasteLyrics(v); } }))}
+                    {#if lyr.lines.length || lyr.plain}
+                        {@render row("clock", lyr.lines.length ? "Re-time by hand" : "Sync these lyrics", tapLeft ? `${tapLeft.done} of ${tapLeft.total} lines timed — keep going` : "play the song and tap as each line begins", act(openTapSync))}
+                    {/if}
+                    {#if canReduce}
+                        {@render row("music", onReduced ? "Back to the original" : "Play with reduced vocals", onReduced ? "the recording as it is" : lyr.reducing ? "making the copy now — a moment" : "a rough centre cancel, made on this phone", act(() => { void (onReduced ? backToOriginal() : reduceVocals()); }))}
+                    {/if}
                     {@render row("refresh", "Forget and look again", "", act(() => { void forgetLyrics(); }))}
                 </div>
             {/if}