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>

package.json

@@ -16,6 +16,7 @@
   "license": "ISC",
   "dependencies": {
     "@radix-ui/react-dialog": "^1.1.15",
+    "@radix-ui/react-dropdown-menu": "^2.1.16",
     "@radix-ui/react-label": "^2.1.8",
     "@radix-ui/react-select": "^2.2.6",
     "@radix-ui/react-slider": "^1.3.6",