feat(app): add TransposeBar collapsible header

app/app/components/transpose-bar.tsx

@@ -0,0 +1,85 @@
+import { useState } from "react";
+import { Button } from "~/components/ui/button";
+import { ChevronUp, ChevronDown, Minus, Plus } from "lucide-react";
+import type { SongMeta } from "~/lib/types";
+
+interface Props {
+  meta: SongMeta;
+  offset: number;
+  onOffsetChange: (offset: number) => void;
+}
+
+export function TransposeBar({ meta, offset, onOffsetChange }: Props) {
+  const [expanded, setExpanded] = useState(true);
+
+  const label = offset === 0
+    ? "±0"
+    : offset > 0
+    ? `+${offset}`
+    : `${offset}`;
+
+  if (!expanded) {
+    return (
+      <div className="flex items-center justify-between px-4 py-2 border-b bg-background sticky top-0">
+        <span className="text-sm font-semibold truncate">{meta.title}</span>
+        <Button
+          variant="ghost"
+          size="icon"
+          className="h-7 w-7"
+          onClick={() => setExpanded(true)}
+        >
+          <ChevronDown className="w-4 h-4" />
+        </Button>
+      </div>
+    );
+  }
+
+  return (
+    <div className="border-b bg-background sticky top-0 px-4 py-3 flex flex-col gap-2">
+      <div className="flex items-start justify-between">
+        <div className="flex flex-col">
+          <span className="font-bold text-base">{meta.title}</span>
+          <span className="text-sm text-muted-foreground">{meta.artist}</span>
+        </div>
+        <Button
+          variant="ghost"
+          size="icon"
+          className="h-7 w-7 shrink-0"
+          onClick={() => setExpanded(false)}
+        >
+          <ChevronUp className="w-4 h-4" />
+        </Button>
+      </div>
+
+      <div className="flex items-center justify-between">
+        <div className="flex gap-3 text-xs text-muted-foreground">
+          {meta.original_key && <span>Key: {meta.original_key}</span>}
+          {meta.capo != null && <span>Capo: {meta.capo}</span>}
+          {meta.tuning && <span>{meta.tuning}</span>}
+        </div>
+
+        <div className="flex items-center gap-2">
+          <Button
+            variant="ghost"
+            size="icon"
+            className="h-8 w-8"
+            onClick={() => onOffsetChange(Math.max(-11, offset - 1))}
+          >
+            <Minus className="w-4 h-4" />
+          </Button>
+          <span className="w-8 text-center text-sm font-mono font-semibold">
+            {label}
+          </span>
+          <Button
+            variant="ghost"
+            size="icon"
+            className="h-8 w-8"
+            onClick={() => onOffsetChange(Math.min(11, offset + 1))}
+          >
+            <Plus className="w-4 h-4" />
+          </Button>
+        </div>
+      </div>
+    </div>
+  );
+}