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/world/mission_marker.gd

@@ -0,0 +1,37 @@
+extends Button
+## Un lieu de la carte : pictogramme du type d'exercice, titre, etoiles
+## gagnees (pleines/vides : la forme porte l'information, pas la couleur) et
+## etat de la competence. Scene autonome : la carte la configure et ecoute
+## `pressed`.
+
+const STAR_FULL := preload("res://assets/interface/star_full.svg")
+const STAR_EMPTY := preload("res://assets/interface/star_empty.svg")
+const TYPE_ICONS := {
+    "multiple_choice": "res://assets/interface/icon_multiple_choice.svg",
+    "sentence_correction": "res://assets/interface/icon_sentence_correction.svg",
+    "dictation": "res://assets/interface/icon_dictation.svg",
+}
+const STAR_SIZE := 24.0
+
+@onready var _type_icon: TextureRect = %TypeIcon
+@onready var _title_label: Label = %TitleLabel
+@onready var _stars_row: HBoxContainer = %StarsRow
+@onready var _status_label: Label = %StatusLabel
+
+
+func setup(mission: Dictionary, stars: int, max_stars: int, status_text: String) -> void:
+    _title_label.text = mission["title"]
+    _status_label.text = status_text
+    _status_label.visible = not status_text.is_empty()
+    _type_icon.texture = load(TYPE_ICONS.get(mission["exercise_type"], TYPE_ICONS["multiple_choice"]))
+
+    for child in _stars_row.get_children():
+        child.queue_free()
+    for i in max_stars:
+        var star := TextureRect.new()
+        star.texture = STAR_FULL if i < stars else STAR_EMPTY
+        star.custom_minimum_size = Vector2(STAR_SIZE, STAR_SIZE)
+        star.expand_mode = TextureRect.EXPAND_IGNORE_SIZE
+        star.stretch_mode = TextureRect.STRETCH_KEEP_ASPECT_CENTERED
+        star.mouse_filter = Control.MOUSE_FILTER_IGNORE
+        _stars_row.add_child(star)