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>

project.godot

@@ -11,7 +11,7 @@ config_version=5
 [application]
 
 config/name="Les Aventuriers des mots"
-config/version="0.2.0"
+config/version="0.3.0"
 run/main_scene="res://scenes/boot/boot.tscn"
 config/features=PackedStringArray("4.6", "Forward Plus")
 config/icon="res://icon.svg"
@@ -25,3 +25,14 @@ ExerciseEngine="*res://autoload/exercise_engine.gd"
 SaveManager="*res://autoload/save_manager.gd"
 AudioManager="*res://autoload/audio_manager.gd"
 SettingsManager="*res://autoload/settings_manager.gd"
+
+[display]
+
+window/size/viewport_width=1152
+window/size/viewport_height=648
+window/stretch/mode="canvas_items"
+window/stretch/aspect="expand"
+
+[gui]
+
+theme/custom="res://assets/interface/theme.tres"