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/backdrop.gd

@@ -0,0 +1,24 @@
+extends Control
+## Decor plein ecran derriere un ecran (illustration vectorielle provisoire,
+## voir assets/CREDITS.md). "forest" pour le CE1, "marsh" pour le CE2.
+
+const TEXTURES := {
+    "forest": "res://assets/environments/forest_backdrop.svg",
+    "marsh": "res://assets/environments/marsh_backdrop.svg",
+}
+
+@export_enum("forest", "marsh") var kind: String = "forest":
+    set(value):
+        kind = value
+        _apply()
+
+@onready var _picture: TextureRect = $Picture
+
+
+func _ready() -> void:
+    _apply()
+
+
+func _apply() -> void:
+    if _picture != null:
+        _picture.texture = load(TEXTURES.get(kind, TEXTURES["forest"]))