feat: add user menu dropdown with settings link and sign out

Replace the plain "Sign out" button in the header with a user icon
that opens a dropdown menu containing Settings and Sign out options.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/superpowers/specs/2026-04-03-user-menu-design.md

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+# User Menu Design
+
+## Summary
+
+Replace the plain "Sign out" button in the header with a user icon that opens a dropdown menu containing Settings and Sign out options. This provides a way to navigate to the Settings page from the header.
+
+## Components
+
+### UserMenu (`src/client/components/UserMenu.tsx`)
+
+New component rendered by `TotalsBar` when authenticated.
+
+**Trigger:** Circular `CircleUser` icon button (Lucide). Styled consistently with surrounding header elements.
+
+**Dropdown:** Absolutely-positioned popover anchored to the right edge, appearing below the icon:
+
+1. **Settings** — `Settings` (gear) icon + "Settings" label, `<Link to="/settings">`
+2. **Divider** — thin horizontal line
+3. **Sign out** — `LogOut` icon + "Sign out" label, calls `logout.mutate()` from `useLogout()`
+
+**Behavior:**
+- Click icon toggles open/close
+- Click outside closes (via `useEffect` with document click listener)
+- Clicking a menu item closes the dropdown
+- Dropdown anchored right so it doesn't overflow viewport
+
+### TotalsBar Changes (`src/client/components/TotalsBar.tsx`)
+
+- When `isAuthenticated`: render `<UserMenu />` in place of the current "Sign out" button
+- When not authenticated: keep the existing "Sign in" link unchanged
+- Remove the `useLogout` hook usage from TotalsBar (moved into UserMenu)
+
+## No Backend Changes
+
+The existing `/api/auth/me` endpoint and `useAuth` hook are sufficient. No username display needed — using a generic user icon.