Add navbar music player with floating ambient panel.

Move ambient audio into a top-nav toggle that opens a retro floating player while keeping playback running when collapsed.

components/layout/header-music-button.tsx

@@ -0,0 +1,42 @@
+"use client";
+
+import { Music2 } from "lucide-react";
+
+import { useAmbientAudioOptional } from "@/components/layout/ambient-audio-provider";
+import { cn } from "@/lib/utils";
+
+export function HeaderMusicButton() {
+  const ambient = useAmbientAudioOptional();
+
+  if (!ambient?.enabled) {
+    return null;
+  }
+
+  const { panelOpen, togglePanel, isPlaying, muted, volume } = ambient;
+  const active = isPlaying && !muted && volume > 0;
+
+  return (
+    <button
+      type="button"
+      data-ambient-audio-trigger
+      aria-label={panelOpen ? "Close music player" : "Open music player"}
+      aria-expanded={panelOpen}
+      aria-pressed={panelOpen}
+      onClick={togglePanel}
+      className={cn(
+        "inline-flex h-10 w-10 items-center justify-center border-2 bg-background/80 shadow-[var(--retro-shadow-sm)] transition-transform",
+        panelOpen
+          ? "border-secondary bg-secondary/15 text-secondary"
+          : "border-border/70 text-foreground hover:border-secondary/50 hover:bg-secondary/10",
+        active && !panelOpen && "ambient-radio-nav-active",
+      )}
+    >
+      <span className="relative flex items-center justify-center">
+        <Music2 aria-hidden="true" className="size-4" />
+        {active ? (
+          <span className="ambient-radio-nav-dot" aria-hidden="true" />
+        ) : null}
+      </span>
+    </button>
+  );
+}