feat(app,player): tile context menu, buffered range on the seek bar, stall spinner

- right-click a library tile to play or resume it, mark it watched or
  unwatched, download it, change its match, or take it off Continue
  watching
- mpv's demuxer-cache-time is shaded on the seek bar, so it shows
  whether a jump lands in what is already read ahead
- paused-for-cache shows a spinner over the picture and in the controls

crates/pstr-app/src/engine.rs

@@ -478,10 +478,12 @@ impl Engine {
     /// Send an event and wake the UI. A closed channel means the window is
     /// gone, which is not an error worth propagating anywhere.
     pub(crate) fn emit(&self, event: Event) {
+        // The two that arrive on mpv's clock rather than on anything happening.
         let ticking = matches!(
             event,
             Event::Player {
-                event: pstr_player::PlayerEvent::Position(_),
+                event: pstr_player::PlayerEvent::Position(_)
+                    | pstr_player::PlayerEvent::Buffered(_),
                 ..
             }
         );