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.tscn

@@ -0,0 +1,59 @@
+[gd_scene load_steps=3 format=3]
+
+[ext_resource type="Script" path="res://scenes/world/mission_marker.gd" id="1"]
+[ext_resource type="Texture2D" path="res://assets/interface/star_empty.svg" id="2"]
+
+[node name="MissionMarker" type="Button"]
+theme_type_variation = &"MarkerButton"
+custom_minimum_size = Vector2(220, 120)
+script = ExtResource("1")
+
+[node name="Content" type="VBoxContainer" parent="."]
+layout_mode = 1
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+grow_horizontal = 2
+grow_vertical = 2
+mouse_filter = 2
+theme_override_constants/separation = 2
+alignment = 1
+
+[node name="Header" type="HBoxContainer" parent="Content"]
+layout_mode = 2
+mouse_filter = 2
+theme_override_constants/separation = 8
+alignment = 1
+
+[node name="TypeIcon" type="TextureRect" parent="Content/Header"]
+unique_name_in_owner = true
+layout_mode = 2
+custom_minimum_size = Vector2(36, 36)
+size_flags_vertical = 4
+mouse_filter = 2
+expand_mode = 1
+stretch_mode = 5
+
+[node name="TitleLabel" type="Label" parent="Content/Header"]
+unique_name_in_owner = true
+layout_mode = 2
+size_flags_horizontal = 3
+mouse_filter = 2
+theme_override_font_sizes/font_size = 18
+autowrap_mode = 3
+max_lines_visible = 3
+text_overrun_behavior = 3
+vertical_alignment = 1
+
+[node name="StarsRow" type="HBoxContainer" parent="Content"]
+unique_name_in_owner = true
+layout_mode = 2
+mouse_filter = 2
+alignment = 1
+
+[node name="StatusLabel" type="Label" parent="Content"]
+unique_name_in_owner = true
+layout_mode = 2
+mouse_filter = 2
+theme_type_variation = &"Small"
+horizontal_alignment = 1