feat: migrate frontend React 18 → Svelte 5 + SvelteKit

Core logic portable (economy, balance, cosmetics, migrateSave) — zero rewrite.
136 tests green, identiques. Backend inchangé.

- Svelte 5 runes stores (game, auth, toast) remplacent Zustand
- SvelteKit adapter-static SPA (dist/ output, fallback index.html)
- Tailwind v4 conservé, design system .gp-* porté
- Transitions natives : slide, fly, scale, fade sur toute l'UI
- Sidebar tabbée (Production/Evolution/Collection) + CollapsiblePanel
- Mobile bottom sheet avec FAB toggle + backdrop blur
- Click particles réactifs Svelte (plus de DOM impératif)
- TadpoleSprite bounce + glow ring au clic
- Guide refait en accordéon, Achievements avec filtres
- a11y : focus-visible, Escape modals, aria-current, aria-labels
- CI/CD adapté (tests + build + rsync)
- Build 504K (vs ~1.2MB React)

Frontend/static/NavBar/SnowOff.svg

@@ -0,0 +1,15 @@
+<svg width="39" height="39" viewBox="0 0 39 39" fill="none" xmlns="http://www.w3.org/2000/svg">
+<!-- Bubbles OFF — inactive state, dashed border -->
+<rect x="1" y="1" width="37" height="37" rx="7" stroke="#202020" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" stroke-dasharray="20 20"/>
+<!-- Big bubble -->
+<circle cx="19" cy="16" r="8" stroke="#202020" stroke-width="2" fill="none"/>
+<circle cx="16" cy="13" r="2" fill="#202020" opacity="0.3"/>
+<!-- Medium bubble -->
+<circle cx="12" cy="28" r="5" stroke="#202020" stroke-width="1.8" fill="none"/>
+<circle cx="10.5" cy="26.5" r="1.2" fill="#202020" opacity="0.25"/>
+<!-- Small bubble -->
+<circle cx="28" cy="26" r="3.5" stroke="#202020" stroke-width="1.5" fill="none"/>
+<circle cx="27" cy="24.8" r="0.9" fill="#202020" opacity="0.25"/>
+<!-- Tiny bubble -->
+<circle cx="30" cy="14" r="2" stroke="#202020" stroke-width="1.2" fill="none"/>
+</svg>

Frontend/static/NavBar/SnowOn.svg

@@ -0,0 +1,15 @@
+<svg width="39" height="39" viewBox="0 0 39 39" fill="none" xmlns="http://www.w3.org/2000/svg">
+<!-- Bubbles ON — active state, filled background -->
+<rect width="39" height="39" rx="8" fill="#202020"/>
+<!-- Big bubble -->
+<circle cx="19" cy="16" r="8" stroke="#8ecae6" stroke-width="2" fill="none"/>
+<circle cx="16" cy="13" r="2" fill="#8ecae6" opacity="0.6"/>
+<!-- Medium bubble -->
+<circle cx="12" cy="28" r="5" stroke="#8ecae6" stroke-width="1.8" fill="none"/>
+<circle cx="10.5" cy="26.5" r="1.2" fill="#8ecae6" opacity="0.5"/>
+<!-- Small bubble -->
+<circle cx="28" cy="26" r="3.5" stroke="#8ecae6" stroke-width="1.5" fill="none"/>
+<circle cx="27" cy="24.8" r="0.9" fill="#8ecae6" opacity="0.5"/>
+<!-- Tiny bubble -->
+<circle cx="30" cy="14" r="2" stroke="#8ecae6" stroke-width="1.2" fill="none"/>
+</svg>