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>

data/localization/fr.json

@@ -17,6 +17,7 @@
   "profile_create.cancel_button": "Retour",
   "profile_create.name_required_error": "Choisis un prénom avant de continuer.",
   "world_map.title": "La forêt des accords",
+  "world_map.guide_hint": "Choisis un lieu pour commencer l'aventure !",
   "world_map.title_ce2": "Le marais des homophones",
   "world_map.review_ce1_button": "Revoir le CE1",
   "world_map.back_to_ce2_button": "Retour au CE2",