Add keyboard play: arrows/WASD/hjkl focus, Enter/Space flips

Focus ring appears on the first key press and hides when the mouse moves.
Enter also triggers Play Again on the win panel.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/screens/screens.odin

@@ -60,7 +60,8 @@ main_menu :: proc() -> Maybe(decks.Language) {
         }
     }
 
-    ui.draw_text_centered("1-3 pick a language  ·  Esc goes back, and quits from here", themes.SCREEN_WIDTH / 2, 660, 18, raylib.Fade(themes.DARK_GRAY, 0.8))
+    ui.draw_text_centered("1-3 pick a language  ·  In game: arrows, WASD or hjkl to move, Enter/Space to flip  ·  Esc back / quit",
+                          themes.SCREEN_WIDTH / 2, 660, 18, raylib.Fade(themes.DARK_GRAY, 0.8))
     return picked
 }