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>

Makefile

@@ -6,7 +6,7 @@ PYRIGHT := $(VENV)/bin/pyright
 PYTHON := $(VENV)/bin/python3
 GD_SOURCES := autoload scripts tests
 
-.PHONY: setup run lint format validate-data test check content-review generate-dictation-audio export-linux export-windows package
+.PHONY: setup run lint format validate-data test check content-review generate-dictation-audio export-linux export-windows package screenshot
 
 setup:
 	tools/setup.sh
@@ -45,3 +45,8 @@ export-windows:
 
 package:
 	tools/package.sh
+
+# Capture d'une scene dans une vraie fenetre (voir tools/screenshot.gd) :
+#   make screenshot SCENE=res://scenes/world/world_map.tscn OUT=/tmp/map.png ARGS="CE2"
+screenshot:
+	$(GODOT_BIN) --path . -s res://tools/screenshot.gd -- $(SCENE) $(OUT) $(ARGS)