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/ui/feedback_panel.gd

@@ -5,6 +5,10 @@ extends Control
 
 signal continue_pressed
 
+const ICON_CORRECT := preload("res://assets/interface/icon_correct.svg")
+const ICON_TRY_AGAIN := preload("res://assets/interface/icon_try_again.svg")
+
+@onready var _icon: TextureRect = %Icon
 @onready var _title_label: Label = %TitleLabel
 @onready var _hint_box: VBoxContainer = %HintBox
 @onready var _hint_label: Label = %HintLabel
@@ -21,6 +25,8 @@ func _ready() -> void:
 
 
 func show_result(result: CorrectionResult, support_level: AttemptFlow.SupportLevel) -> void:
+    _icon.texture = ICON_CORRECT if result.is_correct else ICON_TRY_AGAIN
+    Motion.pop_in(_icon)
     _title_label.text = (
         UIText.get_text("feedback_panel.correct_title")
         if result.is_correct