feat(app,player): page fade, Ctrl+S screenshots

- a page fades in over 140 ms when the page changes, rather than cutting
- Ctrl+S in the player saves the frame, without subtitles, to
  Pictures/proton-stream as "Title  S01E04  12-34.png"

crates/pstr-app/src/ui/shortcuts.rs

@@ -28,6 +28,7 @@ const GROUPS: &[(&str, &[(&str, &str)])] = &[
             ("Space  or  K", "Pause and play"),
             ("←  /  →", "Back 10 s, forward 30 s"),
             ("↑  /  ↓", "Volume"),
+            ("Ctrl S", "Save the frame to Pictures/proton-stream"),
             ("M", "Mute"),
             ("N  /  P", "Next and previous episode"),
             ("[  /  ]", "Slower and faster"),