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/components/CollapsiblePanel.svelte

@@ -0,0 +1,51 @@
+<script lang="ts">
+	import { slide } from 'svelte/transition';
+	import { quintOut } from 'svelte/easing';
+	import type { Snippet } from 'svelte';
+
+	interface Props {
+		title: string;
+		badge?: string;
+		accentClass?: string;
+		defaultOpen?: boolean;
+		children: Snippet;
+	}
+
+	let { title, badge = '', accentClass = '', defaultOpen = true, children }: Props = $props();
+
+	let open = $state(defaultOpen);
+</script>
+
+<div class="gp overflow-hidden">
+	<button
+		class="flex items-center justify-between w-full cursor-pointer group"
+		onclick={() => open = !open}
+	>
+		<div class="flex items-center gap-2">
+			<svg
+				class="w-3 h-3 transition-transform duration-200 text-white/50 group-hover:text-white/80"
+				class:rotate-90={open}
+				viewBox="0 0 24 24"
+				fill="none"
+				stroke="currentColor"
+				stroke-width="3"
+				stroke-linecap="round"
+				stroke-linejoin="round"
+			>
+				<polyline points="9 18 15 12 9 6" />
+			</svg>
+			<span class="gp-title {accentClass}">{title}</span>
+		</div>
+		{#if badge}
+			<span class="gp-label">{badge}</span>
+		{/if}
+	</button>
+
+	{#if open}
+		<div transition:slide={{ duration: 250, easing: quintOut }}>
+			<div class="flex flex-col gap-[var(--spacing-gp-gap)] pt-1">
+				{@render children()}
+			</div>
+		</div>
+	{/if}
+</div>