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>

scripts/ui/motion.gd

@@ -0,0 +1,26 @@
+class_name Motion
+extends RefCounted
+## Petites animations d'interface (apparition, rebond). Toutes respectent le
+## reglage d'animations (cahier des charges §11 : possibilite de les
+## desactiver) : elles ne font alors rien et l'element est affiche tel quel.
+
+
+## Fait grossir `node` depuis rien avec un leger rebond.
+static func pop_in(node: Control, delay: float = 0.0) -> void:
+    if not SettingsManager.animations_enabled:
+        return
+    node.scale = Vector2.ZERO
+    await node.get_tree().process_frame
+    node.pivot_offset = node.size / 2.0
+    var tween := node.create_tween()
+    tween.set_trans(Tween.TRANS_BACK).set_ease(Tween.EASE_OUT)
+    tween.tween_property(node, "scale", Vector2.ONE, 0.4).set_delay(delay)
+
+
+## Fait apparaitre `node` en fondu.
+static func fade_in(node: CanvasItem, duration: float = 0.25) -> void:
+    if not SettingsManager.animations_enabled:
+        return
+    node.modulate.a = 0.0
+    var tween := node.create_tween()
+    tween.tween_property(node, "modulate:a", 1.0, duration)