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_intro.tscn

@@ -1,7 +1,8 @@
-[gd_scene load_steps=3 format=3]
+[gd_scene load_steps=4 format=3]
 
 [ext_resource type="Script" path="res://scenes/world/mission_intro.gd" id="1"]
 [ext_resource type="PackedScene" path="res://scenes/characters/guide_owl.tscn" id="2"]
+[ext_resource type="PackedScene" path="res://scenes/ui/backdrop.tscn" id="backdrop"]
 
 [node name="MissionIntro" type="Control"]
 layout_mode = 3
@@ -12,6 +13,14 @@ grow_horizontal = 2
 grow_vertical = 2
 script = ExtResource("1")
 
+[node name="Backdrop" parent="." instance=ExtResource("backdrop")]
+layout_mode = 1
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+grow_horizontal = 2
+grow_vertical = 2
+
 [node name="Margin" type="MarginContainer" parent="."]
 layout_mode = 1
 anchors_preset = 15
@@ -19,40 +28,45 @@ anchor_right = 1.0
 anchor_bottom = 1.0
 grow_horizontal = 2
 grow_vertical = 2
-theme_override_constants/margin_left = 32
-theme_override_constants/margin_top = 32
-theme_override_constants/margin_right = 32
-theme_override_constants/margin_bottom = 32
+theme_override_constants/margin_left = 48
+theme_override_constants/margin_top = 40
+theme_override_constants/margin_right = 48
+theme_override_constants/margin_bottom = 40
 
-[node name="VBox" type="VBoxContainer" parent="Margin"]
+[node name="Center" type="CenterContainer" parent="Margin"]
 layout_mode = 2
-theme_override_constants/separation = 16
 
-[node name="TitleLabel" type="Label" parent="Margin/VBox"]
-unique_name_in_owner = true
-layout_mode = 2
-theme_override_font_sizes/font_size = 32
-
-[node name="NarrativeLabel" type="Label" parent="Margin/VBox"]
+[node name="Card" type="PanelContainer" parent="Margin/Center"]
+layout_mode = 2
+custom_minimum_size = Vector2(760, 0)
+
+[node name="VBox" type="VBoxContainer" parent="Margin/Center/Card"]
+layout_mode = 2
+theme_override_constants/separation = 18
+
+[node name="TitleLabel" type="Label" parent="Margin/Center/Card/VBox"]
 unique_name_in_owner = true
+theme_type_variation = &"CardTitle"
 layout_mode = 2
+horizontal_alignment = 1
 autowrap_mode = 2
-size_flags_vertical = 3
 
-[node name="Guide" type="Control" parent="Margin/VBox" instance=ExtResource("2")]
+[node name="Guide" type="Control" parent="Margin/Center/Card/VBox" instance=ExtResource("2")]
 unique_name_in_owner = true
 layout_mode = 2
+custom_minimum_size = Vector2(0, 150)
 
-[node name="Buttons" type="HBoxContainer" parent="Margin/VBox"]
+[node name="Buttons" type="HBoxContainer" parent="Margin/Center/Card/VBox"]
 layout_mode = 2
 theme_override_constants/separation = 16
 
-[node name="BackButton" type="Button" parent="Margin/VBox/Buttons"]
+[node name="BackButton" type="Button" parent="Margin/Center/Card/VBox/Buttons"]
 unique_name_in_owner = true
+theme_type_variation = &"SecondaryButton"
 layout_mode = 2
 custom_minimum_size = Vector2(0, 56)
 
-[node name="StartButton" type="Button" parent="Margin/VBox/Buttons"]
+[node name="StartButton" type="Button" parent="Margin/Center/Card/VBox/Buttons"]
 unique_name_in_owner = true
 layout_mode = 2
 size_flags_horizontal = 3