Visual overhaul: theme, illustrations, redesigned screens (0.3.0)

Phase "habillage" of the post-0.1.0 plan. Rendering was reviewed on real
screenshots (new tools/screenshot.gd, `make screenshot`), which also
caught and fixed map markers overlapping and truncated titles.

- Global Godot theme (Atkinson Hyperlegible font, cream/green/gold
  palette, raised buttons, rounded cards, thick keyboard focus).
- Original vector art: forest/marsh backdrops, Plume the owl, stars,
  exercise-type icons, correct/try-again icons, 10 collection items.
- Redesigned world map (markers as cards with icon, title and stars,
  responsive layout), profile screens, mission intro, exercise host,
  rule/feedback panels, reward screen and collection grid.
- Motion helper (pop-in, fade) honouring an animations setting.
- Settings: animations flag; base window 1152x648 with canvas_items.

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

scenes/exercises/multiple_choice.gd

@@ -21,7 +21,8 @@ func load_exercise(exercise: Dictionary) -> void:
     for i in answers.size():
         var button := Button.new()
         button.text = answers[i]
-        button.custom_minimum_size = Vector2(0, 56)
+        button.custom_minimum_size = Vector2(0, 64)
+        button.add_theme_font_size_override("font_size", 28)
         button.pressed.connect(_on_answer_pressed.bind(i))
         _answers_box.add_child(button)