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/utils/platform.ts

@@ -0,0 +1,2 @@
+/** True when running on Linux (WebKitGTK). Used to show the custom title bar. */
+export const IS_LINUX = navigator.platform.toLowerCase().includes('linux');