add AuthCard component for auth pages

src/lib/components/auth/AuthCard.svelte

@@ -0,0 +1,68 @@
+<script lang="ts">
+	import type { Snippet } from 'svelte'
+
+	interface Props {
+		title: string
+		children: Snippet
+		footer?: Snippet
+	}
+
+	let { title, children, footer }: Props = $props()
+</script>
+
+<div class="authCard">
+	<h1 class="title">{title}</h1>
+	<div class="content">
+		{@render children()}
+	</div>
+	{#if footer}
+		<div class="footer">
+			{@render footer()}
+		</div>
+	{/if}
+</div>
+
+<style lang="scss">
+	@use '$src/themes/spacing' as *;
+	@use '$src/themes/typography' as *;
+	@use '$src/themes/effects' as *;
+	@use '$src/themes/layout' as *;
+
+	.authCard {
+		background: var(--card-bg);
+		border-radius: $card-corner;
+		box-shadow: $dialog-elevation;
+		padding: $unit-4x;
+		width: 100%;
+	}
+
+	.title {
+		color: var(--text-primary);
+		font-size: $font-xlarge;
+		font-weight: $bold;
+		margin: 0 0 $unit-3x;
+		text-align: center;
+	}
+
+	.content {
+		display: flex;
+		flex-direction: column;
+		gap: $unit-2x;
+	}
+
+	.footer {
+		margin-top: $unit-3x;
+		text-align: center;
+		color: var(--text-secondary);
+		font-size: $font-small;
+
+		:global(a) {
+			color: var(--accent-blue);
+			text-decoration: none;
+
+			&:hover {
+				text-decoration: underline;
+			}
+		}
+	}
+</style>