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-player/src/queue.rs

@@ -765,6 +765,7 @@ mod tests {
             artist_id: None,
             streamable: true,
             source: Some("test".to_string()),
+            parental_warning: false,
         }
     }