feat(app): media keys and desktop media widget, keep the screen on during a film

- MPRIS on Linux and SMTC on Windows through souvlaki: play/pause, next,
  previous, stop and seeking from media keys and the panel widget, with
  the title, episode, duration and the cached cover as a file:// URL
- the screensaver is held off while a film plays on the player page,
  through the desktop portal with org.freedesktop.ScreenSaver as the
  fallback, and SetThreadExecutionState on Windows. mpv's own
  stop-screensaver has no window to act for under the render API

crates/pstr-app/src/engine.rs

@@ -2008,6 +2008,15 @@ impl Engine {
         });
     }
 
+    /// Where a provider's picture is cached, if it is.
+    pub fn poster_file(&self, url: &str) -> Option<std::path::PathBuf> {
+        let path = self
+            .dirs
+            .poster_cache()
+            .join(format!("{}.img", digest(url)));
+        path.is_file().then_some(path)
+    }
+
     /// Fetch, decode and post one title's artwork.
     ///
     /// Cached on disk under a hash of the URL, so a provider that moves its CDN