feat(ui): add explicit content badge across entire app

Display parental_warning badge (E icon) next to track titles in all
views: TrackRow, QueuePanel, NowPlayingBar, ImmersivePlayer with all
13 visualizer panels, MiniPlayer surfaces, and AlbumDetailView.

Propagate parental_warning field through the full data chain from
Rust backend (qbz-models, queue, commands_v2, session_store, API
server) to frontend stores, adapters, and UI components. Badge uses
CSS mask technique with explicit.svg for theme compatibility.

crates/qbz-models/src/playback.rs

@@ -36,6 +36,9 @@ pub struct QueueTrack {
     /// Source identifier (e.g., "qobuz", "local", "plex")
     #[serde(default)]
     pub source: Option<String>,
+    /// Parental advisory / explicit content
+    #[serde(default)]
+    pub parental_warning: bool,
 }
 
 fn default_streamable() -> bool {