feat(app): up-next card with the next still and a countdown ring, remember the window size

The up-next card shows the next episode's still with the seconds left
running round it, and the still itself plays it. The window opens at
the size and maximized state it closed at; position is left to the
window manager.

crates/pstr-core/src/config.rs

@@ -73,6 +73,11 @@ impl AppDirs {
         Ok(())
     }
 
+    /// The desktop window's size, for the next launch to open at.
+    pub fn window_file(&self) -> PathBuf {
+        self.config.join("window.json")
+    }
+
     /// The share list.
     pub fn shares_file(&self) -> PathBuf {
         self.config.join("shares.json")