feat: add session creation and ID management with localStorage

- Session store with UUID v4 generation and localStorage persistence
- Session ID in URL params (?session=<id>) for deep linking
- "New Chat" button for creating fresh sessions
- Message history persisted per session
- Session title auto-generated from first user message

Closes #11

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

src/lib/stores/sessions.svelte.ts

@@ -0,0 +1,109 @@
+import type { ChatMessage } from '$lib/types';
+
+export interface Session {
+	id: string;
+	title: string;
+	messages: ChatMessage[];
+	createdAt: Date;
+}
+
+const STORAGE_KEY = 'llm-multiverse-sessions';
+const ACTIVE_SESSION_KEY = 'llm-multiverse-active-session';
+
+function loadSessions(): Map<string, Session> {
+	if (typeof localStorage === 'undefined') return new Map();
+	try {
+		const raw = localStorage.getItem(STORAGE_KEY);
+		if (!raw) return new Map();
+		const arr: [string, Session][] = JSON.parse(raw);
+		return new Map(
+			arr.map(([id, s]) => [
+				id,
+				{ ...s, createdAt: new Date(s.createdAt), messages: s.messages.map(m => ({ ...m, timestamp: new Date(m.timestamp) })) }
+			])
+		);
+	} catch {
+		return new Map();
+	}
+}
+
+function saveSessions(sessions: Map<string, Session>) {
+	if (typeof localStorage === 'undefined') return;
+	localStorage.setItem(STORAGE_KEY, JSON.stringify([...sessions.entries()]));
+}
+
+function loadActiveSessionId(): string | null {
+	if (typeof localStorage === 'undefined') return null;
+	return localStorage.getItem(ACTIVE_SESSION_KEY);
+}
+
+function saveActiveSessionId(id: string) {
+	if (typeof localStorage === 'undefined') return;
+	localStorage.setItem(ACTIVE_SESSION_KEY, id);
+}
+
+function createSessionStore() {
+	const sessions = $state<Map<string, Session>>(loadSessions());
+	let activeSessionId = $state<string | null>(loadActiveSessionId());
+
+	function createSession(id?: string): Session {
+		const session: Session = {
+			id: id ?? crypto.randomUUID(),
+			title: 'New Chat',
+			messages: [],
+			createdAt: new Date()
+		};
+		sessions.set(session.id, session);
+		activeSessionId = session.id;
+		saveSessions(sessions);
+		saveActiveSessionId(session.id);
+		return session;
+	}
+
+	function getOrCreateSession(id?: string): Session {
+		if (id && sessions.has(id)) {
+			activeSessionId = id;
+			saveActiveSessionId(id);
+			return sessions.get(id)!;
+		}
+		return createSession(id);
+	}
+
+	function updateMessages(sessionId: string, messages: ChatMessage[]) {
+		const session = sessions.get(sessionId);
+		if (!session) return;
+		session.messages = messages;
+		// Update title from first user message if still default
+		if (session.title === 'New Chat') {
+			const firstUser = messages.find(m => m.role === 'user');
+			if (firstUser) {
+				session.title = firstUser.content.slice(0, 50) + (firstUser.content.length > 50 ? '...' : '');
+			}
+		}
+		sessions.set(sessionId, session);
+		saveSessions(sessions);
+	}
+
+	function switchSession(id: string) {
+		if (sessions.has(id)) {
+			activeSessionId = id;
+			saveActiveSessionId(id);
+		}
+	}
+
+	function getAllSessions(): Session[] {
+		return [...sessions.values()].sort((a, b) => b.createdAt.getTime() - a.createdAt.getTime());
+	}
+
+	return {
+		get activeSessionId() { return activeSessionId; },
+		get activeSession() { return activeSessionId ? sessions.get(activeSessionId) ?? null : null; },
+		createSession,
+		getOrCreateSession,
+		updateMessages,
+		switchSession,
+		getAllSessions
+	};
+}
+
+export const sessionStore = createSessionStore();