feat(13.1): add UserMenu component with avatar, Settings link, and Sign Out

- Install @radix-ui/react-dropdown-menu and add shadcn/ui dropdown-menu component
- Create src/components/user-menu.tsx: avatar button showing user initial,
  DropdownMenu with name/email label, Settings link (/app/settings), Sign Out
  (calls signOut with redirect to /login)
- Replace placeholder div in src/app/app/layout.tsx with <UserMenu />
- Remove now-redundant standalone Settings link from nav bar (Settings is in dropdown)
- Mark task 13.1 done in openspec/changes/user-accounts/tasks.md

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

src/app/app/layout.tsx

@@ -2,7 +2,7 @@
 
 import { SessionProvider } from 'next-auth/react';
 import Link from 'next/link';
-import { Settings } from 'lucide-react';
+import { UserMenu } from '@/components/user-menu';
 
 /**
  * Protected app layout.
@@ -12,10 +12,7 @@ import { Settings } from 'lucide-react';
  *
  * Contains a minimal top nav bar with:
  *  - App title / home link
- *  - Settings link (→ /app/settings)
- *  - Placeholder slot for the user menu (task 13.1)
- *
- * The full user-menu dropdown (avatar, sign-out, etc.) is wired up in task 13.1.
+ *  - UserMenu component (avatar, settings link, sign-out)
  */
 export default function AppLayout({
   children,
@@ -24,7 +21,6 @@ export default function AppLayout({
 }>) {
   return (
     <SessionProvider>
-      {/* Minimal nav bar — user menu dropdown added in task 13.1 */}
       <header className="fixed top-0 inset-x-0 z-50 h-10 flex items-center justify-between px-4 bg-background border-b border-border">
         <Link
           href="/app"
@@ -34,19 +30,7 @@ export default function AppLayout({
         </Link>
 
         <nav className="flex items-center gap-2">
-          {/* Settings link */}
-          <Link
-            href="/app/settings"
-            className="flex items-center gap-1.5 px-2 py-1 text-xs text-muted-foreground hover:text-foreground rounded hover:bg-secondary/50 transition-colors"
-          >
-            <Settings className="w-3.5 h-3.5" />
-            Settings
-          </Link>
-
-          {/* User menu placeholder — implemented in task 13.1 */}
-          <div className="w-7 h-7 rounded-full bg-secondary/50 flex items-center justify-center text-xs text-muted-foreground" aria-label="User menu (coming soon)">
-            ?
-          </div>
+          <UserMenu />
         </nav>
       </header>