feat: add /admin/login page (v0.14.0)

Password form at /admin/login that calls POST /api/v1/admin/login and
redirects to /admin on success. Admin dashboard now redirects to
/admin/login instead of /join when unauthenticated. Test guide updated.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/admin/+page.svelte

@@ -47,7 +47,7 @@
 		const token = getToken();
 		const role = getRole();
 		if (!token || role !== 'admin') {
-			goto('/join');
+			goto('/admin/login');
 			return;
 		}
 		await reload();

frontend/src/routes/admin/login/+page.svelte

@@ -0,0 +1,68 @@
+<script lang="ts">
+	import { goto } from '$app/navigation';
+	import { api, ApiError } from '$lib/api';
+	import { setAuth, getRole } from '$lib/auth';
+	import { browser } from '$app/environment';
+
+	// Already logged in as admin → go straight to dashboard
+	if (browser && getRole() === 'admin') {
+		goto('/admin');
+	}
+
+	let password = $state('');
+	let error = $state('');
+	let loading = $state(false);
+
+	async function handleLogin() {
+		if (!password) return;
+		loading = true;
+		error = '';
+		try {
+			const res = await api.post<{ jwt: string }>('/admin/login', { password });
+			// Admin sessions have no PIN; pass null so setAuth doesn't overwrite a guest PIN
+			setAuth(res.jwt, null, '');
+			goto('/admin');
+		} catch (e) {
+			if (e instanceof ApiError) {
+				error = e.message;
+			} else {
+				error = 'Ein Fehler ist aufgetreten.';
+			}
+		} finally {
+			loading = false;
+		}
+	}
+</script>
+
+<div class="flex min-h-screen items-center justify-center bg-gray-50 px-4">
+	<div class="w-full max-w-sm">
+		<h1 class="mb-2 text-center text-2xl font-bold text-gray-900">Admin-Login</h1>
+		<p class="mb-6 text-center text-gray-500 text-sm">Nur für Veranstalter</p>
+
+		<form onsubmit={(e) => { e.preventDefault(); handleLogin(); }}>
+			<input
+				type="password"
+				bind:value={password}
+				placeholder="Passwort"
+				autocomplete="current-password"
+				class="mb-3 w-full rounded-lg border border-gray-300 px-4 py-3 text-lg focus:border-blue-500 focus:outline-none focus:ring-2 focus:ring-blue-200"
+			/>
+
+			{#if error}
+				<p class="mb-3 text-sm text-red-600">{error}</p>
+			{/if}
+
+			<button
+				type="submit"
+				disabled={loading || !password}
+				class="w-full rounded-lg bg-blue-600 px-4 py-3 text-lg font-medium text-white transition hover:bg-blue-700 disabled:opacity-50"
+			>
+				{loading ? 'Wird angemeldet…' : 'Anmelden'}
+			</button>
+		</form>
+
+		<p class="mt-4 text-center text-sm text-gray-500">
+			<a href="/join" class="text-blue-600 hover:underline">Zurück zum Event</a>
+		</p>
+	</div>
+</div>