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/svg/cosmetics/particles-gold.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 32 32"><g fill="#fbbf24" opacity="0.6"><circle cx="6" cy="8" r="1.2"/><circle cx="26" cy="6" r="1"/><circle cx="4" cy="20" r="0.8"/><circle cx="28" cy="22" r="1.2"/><circle cx="10" cy="28" r="1"/><circle cx="22" cy="30" r="0.8"/><circle cx="16" cy="2" r="1"/><circle cx="2" cy="14" r="0.8"/><circle cx="30" cy="14" r="1"/></g></svg>