feat(titlebar): custom Linux title bar with now-playing display

- set_decorations(false) on Linux at startup via #[cfg(target_os = "linux")]
- New TitleBar component: drag region + minimize/maximize/close buttons
- Currently playing track (▶/⏸ + artist – title) shown in the center
- app-shell grid gains a 32px titlebar row when data-titlebar is set
- IS_LINUX utility constant via navigator.platform
- macOS and Windows are completely unaffected

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/TitleBar.tsx

@@ -0,0 +1,55 @@
+import React from 'react';
+import { getCurrentWindow } from '@tauri-apps/api/window';
+import { X, Minus, Square } from 'lucide-react';
+import { usePlayerStore } from '../store/playerStore';
+
+const win = getCurrentWindow();
+
+export default function TitleBar() {
+  const currentTrack = usePlayerStore(s => s.currentTrack);
+  const isPlaying = usePlayerStore(s => s.isPlaying);
+
+  return (
+    <div className="titlebar" data-tauri-drag-region>
+      <span className="titlebar-title" data-tauri-drag-region>Psysonic</span>
+
+      <div className="titlebar-track" data-tauri-drag-region>
+        {currentTrack && (
+          <>
+            <span className="titlebar-track-state">{isPlaying ? '▶' : '⏸'}</span>
+            <span className="titlebar-track-text truncate">
+              {currentTrack.artist && `${currentTrack.artist} – `}{currentTrack.title}
+            </span>
+          </>
+        )}
+      </div>
+
+      <div className="titlebar-controls">
+        <button
+          className="titlebar-btn titlebar-btn-minimize"
+          onClick={() => win.minimize()}
+          data-tooltip="Minimize"
+          data-tooltip-pos="bottom"
+        >
+          <Minus size={10} />
+        </button>
+        <button
+          className="titlebar-btn titlebar-btn-maximize"
+          onClick={() => win.toggleMaximize()}
+          data-tooltip="Maximize"
+          data-tooltip-pos="bottom"
+        >
+          <Square size={9} />
+        </button>
+        <button
+          className="titlebar-btn titlebar-btn-close"
+          onClick={() => win.close()}
+          data-tooltip="Close"
+          data-tooltip-pos="bottom"
+        >
+          <X size={12} />
+        </button>
+      </div>
+    </div>
+  );
+}