feat(web): scaffold Next.js 16 dashboard with design system and auth client (#82)

Co-authored-by: Jason Woltje <jason@diversecanvas.com>
Co-committed-by: Jason Woltje <jason@diversecanvas.com>

apps/web/src/components/layout/app-shell.tsx

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+import type { ReactNode } from 'react';
+import { Sidebar } from './sidebar';
+import { Topbar } from './topbar';
+
+interface AppShellProps {
+  children: ReactNode;
+}
+
+export function AppShell({ children }: AppShellProps): React.ReactElement {
+  return (
+    <div className="min-h-screen">
+      <Sidebar />
+      <div className="pl-sidebar">
+        <Topbar />
+        <main className="p-6">{children}</main>
+      </div>
+    </div>
+  );
+}

apps/web/src/components/layout/sidebar.tsx

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+'use client';
+
+import Link from 'next/link';
+import { usePathname } from 'next/navigation';
+import { cn } from '@/lib/cn';
+
+interface NavItem {
+  label: string;
+  href: string;
+  icon: string;
+}
+
+const navItems: NavItem[] = [
+  { label: 'Chat', href: '/chat', icon: '💬' },
+  { label: 'Tasks', href: '/tasks', icon: '📋' },
+  { label: 'Projects', href: '/projects', icon: '📁' },
+  { label: 'Settings', href: '/settings', icon: '⚙️' },
+];
+
+export function Sidebar(): React.ReactElement {
+  const pathname = usePathname();
+
+  return (
+    <aside className="fixed left-0 top-0 z-30 flex h-screen w-sidebar flex-col border-r border-surface-border bg-surface-card">
+      <div className="flex h-14 items-center px-4">
+        <Link href="/" className="text-lg font-semibold text-text-primary">
+          Mosaic
+        </Link>
+      </div>
+
+      <nav className="flex-1 space-y-1 px-2 py-2">
+        {navItems.map((item) => {
+          const isActive = pathname === item.href || pathname.startsWith(`${item.href}/`);
+          return (
+            <Link
+              key={item.href}
+              href={item.href}
+              className={cn(
+                'flex items-center gap-3 rounded-lg px-3 py-2 text-sm transition-colors',
+                isActive
+                  ? 'bg-blue-600/20 text-blue-400'
+                  : 'text-text-secondary hover:bg-surface-elevated hover:text-text-primary',
+              )}
+            >
+              <span className="text-base" aria-hidden="true">
+                {item.icon}
+              </span>
+              {item.label}
+            </Link>
+          );
+        })}
+      </nav>
+
+      <div className="border-t border-surface-border p-4">
+        <p className="text-xs text-text-muted">Mosaic Stack v0.0.4</p>
+      </div>
+    </aside>
+  );
+}

apps/web/src/components/layout/topbar.tsx

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+'use client';
+
+import { useSession, signOut } from '@/lib/auth-client';
+
+export function Topbar(): React.ReactElement {
+  const { data: session } = useSession();
+
+  return (
+    <header className="sticky top-0 z-20 flex h-14 items-center justify-between border-b border-surface-border bg-surface-card/80 px-6 backdrop-blur-sm">
+      <div />
+
+      <div className="flex items-center gap-4">
+        {session?.user ? (
+          <>
+            <span className="text-sm text-text-secondary">
+              {session.user.name ?? session.user.email}
+            </span>
+            <button
+              type="button"
+              onClick={() => signOut()}
+              className="rounded-md px-3 py-1.5 text-sm text-text-muted transition-colors hover:bg-surface-elevated hover:text-text-primary"
+            >
+              Sign out
+            </button>
+          </>
+        ) : (
+          <span className="text-sm text-text-muted">Not signed in</span>
+        )}
+      </div>
+    </header>
+  );
+}