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>

assets/interface/items/item_plume_doree.svg

@@ -0,0 +1,5 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="160" height="160" viewBox="0 0 160 160">
+<circle cx="80" cy="80" r="74" fill="#FFF1C9"/>
+<circle cx="80" cy="80" r="74" fill="none" stroke="#E9CF8C" stroke-width="5"/>
+<g><path d="M110 36 C 60 40 40 90 44 126 C 74 116 108 84 110 36 Z" fill="#FFC83D" stroke="#D98E00" stroke-width="5" stroke-linejoin="round"/><path d="M108 40 C 84 66 64 96 46 124" fill="none" stroke="#D98E00" stroke-width="5" stroke-linecap="round"/><path d="M62 100 l-14 6 M74 84 l-14 6 M88 68 l-12 4" stroke="#D98E00" stroke-width="4" stroke-linecap="round"/></g>
+</svg>