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>

scenes/characters/guide_owl.gd

@@ -1,15 +1,23 @@
 extends Control
-## Plume, le hibou guide : affiche une remarque ou un indice sous forme de
-## texte. Portrait provisoire (forme simple, voir assets/CREDITS.md), a
-## remplacer fichier par fichier sans toucher ce script.
+## Plume, le hibou guide : affiche une remarque ou un indice dans une bulle.
+## Illustration vectorielle provisoire (voir assets/CREDITS.md), a remplacer
+## fichier par fichier sans toucher ce script.
 
+@onready var _bubble: Control = $HBox/Bubble
 @onready var _dialogue_label: Label = %DialogueLabel
 
 
+func _ready() -> void:
+    _bubble.visible = _dialogue_label.visible
+
+
 func say(text: String) -> void:
     _dialogue_label.text = text
     _dialogue_label.visible = true
+    _bubble.visible = true
+    Motion.fade_in(_bubble)
 
 
 func clear() -> void:
     _dialogue_label.visible = false
+    _bubble.visible = false