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/GameSync.svelte

@@ -0,0 +1,28 @@
+<script lang="ts">
+	import { onMount } from 'svelte';
+	import { authStore } from '$lib/stores/auth.svelte';
+	import { gameStore } from '$lib/stores/game.svelte';
+	import {
+		loadFromServer,
+		startAutoSave,
+		stopAutoSave,
+		setupVisibilitySync,
+	} from '$lib/save-sync';
+
+	onMount(async () => {
+		// Init auth
+		await authStore.init();
+
+		// Load save or init guest
+		if (authStore.user) {
+			const loaded = await loadFromServer();
+			if (!loaded && !gameStore.ready) {
+				gameStore.initGuest();
+			}
+			startAutoSave();
+			setupVisibilitySync();
+		} else {
+			gameStore.initGuest();
+		}
+	});
+</script>