feat(frontend): implement API client and auth module

Adds the base fetch wrapper (client.ts) with JWT auth headers,
automatic token refresh on 401 with request deduplication, and
typed ApiError. Adds auth.ts with login/logout/refresh/listSessions/
terminateSession. Adds authStore (stores/auth.ts) persisted to
localStorage.

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

frontend/src/lib/stores/auth.ts

@@ -0,0 +1,29 @@
+import { writable, derived } from 'svelte/store';
+import type { User } from '$lib/api/types';
+
+export interface AuthState {
+	accessToken: string | null;
+	refreshToken: string | null;
+	user: User | null;
+}
+
+const initial: AuthState = { accessToken: null, refreshToken: null, user: null };
+
+function loadStored(): AuthState {
+	if (typeof localStorage === 'undefined') return initial;
+	try {
+		return JSON.parse(localStorage.getItem('auth') ?? 'null') ?? initial;
+	} catch {
+		return initial;
+	}
+}
+
+export const authStore = writable<AuthState>(loadStored());
+
+authStore.subscribe((state) => {
+	if (typeof localStorage !== 'undefined') {
+		localStorage.setItem('auth', JSON.stringify(state));
+	}
+});
+
+export const isAuthenticated = derived(authStore, ($auth) => !!$auth.accessToken);