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/data/item_icons.gd

@@ -0,0 +1,20 @@
+class_name ItemIcons
+extends RefCounted
+## Illustration d'un objet de la collection, resolue par identifiant dans un
+## repertoire fixe (CLAUDE.md : jamais de chemin libre dans les donnees). Un
+## objet sans illustration retombe sur un cadenas : jamais d'erreur a l'ecran.
+
+const ITEM_DIR := "res://assets/interface/items"
+const FALLBACK := "res://assets/interface/icon_locked.svg"
+const LOCKED := "res://assets/interface/icon_locked.svg"
+
+
+static func texture_for(item_id: String) -> Texture2D:
+    var path := "%s/%s.svg" % [ITEM_DIR, item_id]
+    if ResourceLoader.exists(path):
+        return load(path)
+    return load(FALLBACK)
+
+
+static func locked_texture() -> Texture2D:
+    return load(LOCKED)