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/profiles/profile_select.gd

@@ -41,14 +41,17 @@ func _refresh_profile_list() -> void:
 
 func _build_profile_row(summary: Dictionary) -> Control:
     var row := HBoxContainer.new()
+    row.add_theme_constant_override("separation", 10)
 
     var select_button := Button.new()
     select_button.text = "%s (%s)" % [summary["display_name"], summary["school_level"]]
     select_button.size_flags_horizontal = Control.SIZE_EXPAND_FILL
+    select_button.custom_minimum_size = Vector2(0, 56)
     select_button.pressed.connect(_on_profile_selected.bind(summary["profile_id"]))
     row.add_child(select_button)
 
     var delete_button := Button.new()
+    delete_button.theme_type_variation = &"SecondaryButton"
     delete_button.text = UIText.get_text("profile_select.delete_button")
     delete_button.pressed.connect(_on_delete_requested.bind(summary["profile_id"]))
     row.add_child(delete_button)