Fix playlists route matching /play and improve playlists page UI

- Add isPlayerRoute() so /playlists no longer hides header and bottom nav
- Add mobile back navigation for playlists, favourites, and history
- Redesign PlaylistsPage with PageHeader, empty state, and card list

Co-authored-by: truewhile <truewhile@users.noreply.github.com>

web/src/components/layoutNavigation.ts

@@ -76,6 +76,11 @@ export const MOBILE_BOTTOM_NAV_ITEMS: MobileBottomNavItem[] = [
 
 const ROOT_MEDIA_PATHS = new Set(['/', '/libraries', '/favourites', '/playlists', '/history'])
 
+/** True only for the fullscreen player route (`/play` or `/play/:id`), not `/playlists`. */
+export function isPlayerRoute(pathname: string): boolean {
+  return pathname === '/play' || pathname.startsWith('/play/')
+}
+
 export function isRootMediaPath(pathname: string): boolean {
   return ROOT_MEDIA_PATHS.has(pathname)
 }
@@ -84,6 +89,9 @@ export function resolveHeaderBack(pathname: string): HeaderBackTarget | null {
   if (pathname.startsWith('/library/')) {
     return { to: '/libraries', label: '媒体库' }
   }
+  if (pathname === '/playlists' || pathname === '/favourites' || pathname === '/history') {
+    return { to: '/', label: '首页' }
+  }
   if (pathname.startsWith('/playlist/')) {
     return { to: '/playlists', label: '播放列表' }
   }
@@ -118,7 +126,7 @@ export function resolveHeaderBack(pathname: string): HeaderBackTarget | null {
 }
 
 export function shouldShowMobileBottomNav(pathname: string): boolean {
-  if (pathname.startsWith('/play')) return false
+  if (isPlayerRoute(pathname)) return false
   return (
     pathname === '/' ||
     pathname === '/libraries' ||