fix: refactor store to singleton class pattern (s.subscribe fix)

Exported $state proxies were confused with Svelte stores by SvelteKit
runtime, causing "s.subscribe is not a function" on /jeu.

Fix: encapsulate all $state fields in a Game class, export singleton.
Components import { game } and access game.state, game.click(), etc.
Class fields are proper $state — no raw proxy exported.

Frontend/src/lib/components/CosmeticsPanel.svelte

@@ -1,7 +1,7 @@
 <script lang="ts">
 	import { fly } from 'svelte/transition';
 	import { quintOut } from 'svelte/easing';
-	import { state, equipCosmetic, unequipCosmetic } from '$lib/stores/game.svelte';
+	import { game } from '$lib/stores/game.svelte';
 	import { COSMETICS, type CosmeticSlot } from '$lib/core/cosmetics';
 	import CollapsiblePanel from './CollapsiblePanel.svelte';
 
@@ -13,8 +13,8 @@
 	};
 	const SLOT_ORDER: CosmeticSlot[] = ['hat', 'eyes', 'body', 'tail', 'accessory'];
 
-	let inventory = $derived(state.cosmeticInventory);
-	let equipped = $derived(state.cosmeticEquipped);
+	let inventory = $derived(game.state.cosmeticInventory);
+	let equipped = $derived(game.state.cosmeticEquipped);
 	let ownedCosmetics = $derived(COSMETICS.filter((c) => inventory.includes(c.id)));
 </script>
 
@@ -40,7 +40,7 @@
 								<span class="gp-label">{cos.description}</span>
 							</div>
 							<button
-								onclick={() => isEquipped ? unequipCosmetic(slot) : equipCosmetic(cos.id)}
+								onclick={() => isEquipped ? game.unequipCosmetic(slot) : game.equipCosmetic(cos.id)}
 								class="gp-btn {isEquipped ? 'gp-btn--disabled' : 'gp-btn--buy'}"
 							>
 								{isEquipped ? 'Retirer' : 'Equiper'}