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/src/lib/stores/auth.svelte.ts

@@ -0,0 +1,64 @@
+// auth.svelte.ts — Auth store (Svelte 5 runes)
+// Cookie-based auth with SuperOAuth PKCE
+
+import { apiFetch } from '$lib/api';
+
+export interface User {
+	id: number;
+	nickname: string;
+	avatar_url?: string;
+	[key: string]: unknown;
+}
+
+let user = $state<User | null>(null);
+let loading = $state(true);
+
+async function refresh() {
+	try {
+		const data = await apiFetch('/auth/me');
+		user = data as User;
+	} catch {
+		user = null;
+	}
+}
+
+async function init() {
+	await refresh();
+	loading = false;
+
+	// Listen for expired session
+	if (typeof window !== 'undefined') {
+		window.addEventListener('auth:expired', () => {
+			user = null;
+		});
+	}
+}
+
+async function logout() {
+	try {
+		await apiFetch('/auth/logout', { method: 'POST' });
+	} catch {
+		// ignore
+	}
+	user = null;
+}
+
+async function editUser(updatedFields: Record<string, unknown>) {
+	if (!user) return;
+	const data = await apiFetch(`/users/${user.id}`, {
+		method: 'PUT',
+		body: JSON.stringify(updatedFields),
+	});
+	if (data?.user) {
+		user = { ...user, ...data.user };
+	}
+}
+
+export const authStore = {
+	get user() { return user; },
+	get loading() { return loading; },
+	init,
+	refresh,
+	logout,
+	editUser,
+};