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/routes/login/+page.svelte

@@ -0,0 +1,54 @@
+<script lang="ts">
+	import { goto } from '$app/navigation';
+	import { fly, scale } from 'svelte/transition';
+	import { quintOut, backOut } from 'svelte/easing';
+	import { authStore } from '$lib/stores/auth.svelte';
+	import { buildAuthUrl, saveVerifier } from '$lib/oauth';
+
+	const PROVIDERS = [
+		{ id: 'discord', label: 'Discord', emoji: '🎮', color: '#5865F2' },
+		{ id: 'github',  label: 'GitHub',  emoji: '🐙', color: '#333' },
+		{ id: 'google',  label: 'Google',  emoji: '🌐', color: '#4285f4' },
+		{ id: 'twitch',  label: 'Twitch',  emoji: '🎬', color: '#9146FF' },
+	];
+
+	$effect(() => {
+		if (authStore.user) goto('/', { replaceState: true });
+	});
+
+	async function handleLogin(provider: string) {
+		const redirectUri = `${window.location.origin}/callback`;
+		const { url, verifier } = await buildAuthUrl(redirectUri, provider);
+		saveVerifier(verifier);
+		window.location.href = url;
+	}
+</script>
+
+<svelte:head>
+	<title>Connexion — Clickerz</title>
+</svelte:head>
+
+<section>
+	<div class="containererror">
+		<div in:scale={{ duration: 400, start: 0.7, easing: backOut }}>
+			<img src="/svg/tadpole.svg" alt="" class="w-24 h-24 mx-auto mb-2 opacity-60" />
+		</div>
+		<h1 in:fly={{ y: 20, delay: 100, duration: 400, easing: quintOut }}>Connexion</h1>
+		<p class="message" in:fly={{ y: 15, delay: 200, duration: 400, easing: quintOut }}>
+			Connecte-toi pour sauvegarder ta progression.
+		</p>
+		<div class="flex flex-col gap-3 mt-6 w-full max-w-xs mx-auto">
+			{#each PROVIDERS as p, i}
+				<button
+					class="btn-return flex items-center justify-center gap-2 py-3! text-base!"
+					onclick={() => handleLogin(p.id)}
+					type="button"
+					in:fly={{ y: 20, delay: 300 + i * 80, duration: 300, easing: quintOut }}
+				>
+					<span class="text-xl">{p.emoji}</span>
+					Continuer avec {p.label}
+				</button>
+			{/each}
+		</div>
+	</div>
+</section>