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/ui/rule_panel.tscn

@@ -31,7 +31,7 @@ grow_vertical = 2
 
 [node name="Panel" type="PanelContainer" parent="CenterContainer"]
 layout_mode = 2
-custom_minimum_size = Vector2(480, 0)
+custom_minimum_size = Vector2(560, 0)
 
 [node name="Margin" type="MarginContainer" parent="CenterContainer/Panel"]
 layout_mode = 2
@@ -46,8 +46,9 @@ theme_override_constants/separation = 16
 
 [node name="TitleLabel" type="Label" parent="CenterContainer/Panel/Margin/VBox"]
 unique_name_in_owner = true
+theme_type_variation = &"CardTitle"
 layout_mode = 2
-theme_override_font_sizes/font_size = 28
+horizontal_alignment = 1
 
 [node name="StatementLabel" type="Label" parent="CenterContainer/Panel/Margin/VBox"]
 unique_name_in_owner = true