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.tscn

@@ -1,15 +1,16 @@
-[gd_scene load_steps=2 format=3]
+[gd_scene load_steps=3 format=3]
 
 [ext_resource type="Script" path="res://scenes/characters/guide_owl.gd" id="1"]
+[ext_resource type="Texture2D" path="res://assets/characters/owl_plume.svg" id="2"]
 
 [node name="GuideOwl" type="Control"]
 layout_mode = 3
 size_flags_horizontal = 3
-custom_minimum_size = Vector2(0, 96)
+custom_minimum_size = Vector2(0, 112)
 script = ExtResource("1")
 
 [node name="HBox" type="HBoxContainer" parent="."]
-layout_mode = 2
+layout_mode = 1
 anchors_preset = 15
 anchor_right = 1.0
 anchor_bottom = 1.0
@@ -17,15 +18,19 @@ grow_horizontal = 2
 grow_vertical = 2
 theme_override_constants/separation = 12
 
-[node name="Portrait" type="ColorRect" parent="HBox"]
+[node name="Portrait" type="TextureRect" parent="HBox"]
 layout_mode = 2
-custom_minimum_size = Vector2(80, 80)
-color = Color(0.56, 0.75, 0.42, 1)
+custom_minimum_size = Vector2(104, 104)
+size_flags_vertical = 4
+texture = ExtResource("2")
+expand_mode = 1
+stretch_mode = 5
 
 [node name="Bubble" type="PanelContainer" parent="HBox"]
+theme_type_variation = &"SoftCard"
 layout_mode = 2
 size_flags_horizontal = 3
-size_flags_vertical = 1
+size_flags_vertical = 4
 
 [node name="DialogueLabel" type="Label" parent="HBox/Bubble"]
 unique_name_in_owner = true