feat: add session history sidebar with delete and navigation

- SessionSidebar component listing past sessions sorted by recency
- Session title preview and relative date display
- Click to switch sessions, delete with confirmation
- Added deleteSession method to session store
- Integrated sidebar into chat page layout

Closes #12

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

src/lib/components/SessionSidebar.svelte

@@ -0,0 +1,81 @@
+<script lang="ts">
+	import { sessionStore } from '$lib/stores/sessions.svelte';
+
+	let {
+		onSelectSession,
+		onNewChat
+	}: { onSelectSession: (id: string) => void; onNewChat: () => void } = $props();
+
+	let confirmDeleteId: string | null = $state(null);
+
+	const sessions = $derived(sessionStore.getAllSessions());
+	const activeId = $derived(sessionStore.activeSessionId);
+
+	function handleDelete(e: MouseEvent, id: string) {
+		e.stopPropagation();
+		if (confirmDeleteId === id) {
+			sessionStore.deleteSession(id);
+			confirmDeleteId = null;
+			if (sessionStore.activeSessionId) {
+				onSelectSession(sessionStore.activeSessionId);
+			}
+		} else {
+			confirmDeleteId = id;
+		}
+	}
+
+	function formatDate(date: Date): string {
+		const now = new Date();
+		const diff = now.getTime() - date.getTime();
+		const days = Math.floor(diff / (1000 * 60 * 60 * 24));
+		if (days === 0) return 'Today';
+		if (days === 1) return 'Yesterday';
+		if (days < 7) return `${days} days ago`;
+		return date.toLocaleDateString();
+	}
+</script>
+
+<aside class="flex h-full w-64 flex-col border-r border-gray-200 bg-gray-50">
+	<div class="border-b border-gray-200 p-3">
+		<button
+			type="button"
+			onclick={onNewChat}
+			class="w-full rounded-lg bg-blue-600 px-3 py-2 text-sm font-medium text-white hover:bg-blue-700"
+		>
+			+ New Chat
+		</button>
+	</div>
+
+	<div class="flex-1 overflow-y-auto">
+		{#if sessions.length === 0}
+			<p class="p-4 text-center text-sm text-gray-400">No sessions yet</p>
+		{:else}
+			{#each sessions as session (session.id)}
+				<div
+					role="button"
+					tabindex="0"
+					onclick={() => onSelectSession(session.id)}
+					onkeydown={(e) => { if (e.key === 'Enter') onSelectSession(session.id); }}
+					class="group flex w-full cursor-pointer items-start gap-2 border-b border-gray-100 px-3 py-3 text-left hover:bg-gray-100
+						{activeId === session.id ? 'bg-blue-50 border-l-2 border-l-blue-500' : ''}"
+				>
+					<div class="min-w-0 flex-1">
+						<p class="truncate text-sm font-medium text-gray-900">{session.title}</p>
+						<p class="mt-0.5 text-xs text-gray-500">{formatDate(session.createdAt)}</p>
+					</div>
+					<button
+						type="button"
+						onclick={(e) => handleDelete(e, session.id)}
+						class="shrink-0 rounded p-1 text-xs opacity-0 group-hover:opacity-100
+							{confirmDeleteId === session.id
+							? 'bg-red-100 text-red-600'
+							: 'text-gray-400 hover:bg-gray-200 hover:text-gray-600'}"
+						title={confirmDeleteId === session.id ? 'Click again to confirm' : 'Delete session'}
+					>
+						{confirmDeleteId === session.id ? '✓' : '✕'}
+					</button>
+				</div>
+			{/each}
+		{/if}
+	</div>
+</aside>