Rebuild ProfileMenu on the registry DropdownMenu per the Nebari header recipe

[jbouder]

Summary

frontend/src/components/layout/ProfileMenu.tsx is a hand-rolled popover (custom open state, onBlur/Escape handling, role="menu" div, Tab-based focus order). The canonical Nebari app-header recipe (.claude/skills/nebari-ui/SKILL.md, "Profile / account menu" and "Theme picker") builds the profile menu from the registry DropdownMenu (modal={false}, DropdownMenuTrigger variant="ghost", DropdownMenuContent align="end" className="w-[248px] p-2") with the Light/Dark/System picker as a Base UI Menu.RadioGroup / Menu.RadioItem (closeOnClick={false}) wired to useTheme().

The registry re-sync PR applied the mechanical parts (248px width, token-based shadow, ThemeMode from @nebari/use-theme-preference, text-sign-out-foreground) but left the composition alone because it is a structural change.

Motivation

• One header across Nebari apps: keyboard behaviour (arrow-key navigation, typeahead, focus return) and open/close semantics should come from the shared DropdownMenu rather than a bespoke implementation.
• Removes ~100 lines of focus/blur bookkeeping that the registry primitive already handles.
• The theme picker should read/write theme state via useTheme() directly instead of props threaded from App through Layout.

Acceptance Criteria

• ProfileMenu renders DropdownMenu modal={false} + DropdownMenuTrigger / DropdownMenuContent from @/components/ui/dropdown-menu (248px, p-2), keeping the avatar/initial + name + ChevronDown trigger.
• Theme picker is a Menu.RadioGroup of three Menu.RadioItems exposing role="menuitemradio" + aria-checked, with closeOnClick={false}, wired to useTheme() from @/hooks/theme-provider.
• Layout no longer needs themeMode / onThemeChange props (it can keep isDarkMode for the logo, or read useTheme() itself).
• Sign-out item uses DropdownMenuItem with text-sign-out-foreground (nebi is a Wails desktop app in local mode; the item stays conditional on team mode as today).
• ProfileMenu.test.tsx and the e2e selectTheme helper are updated for menu keyboard semantics (arrow keys instead of Tab between items) and still pass.

Out of Scope

• Adding a notifications bell (nebi has no in-app notification feed).
• Introducing an avatar primitive; the registry does not publish @nebari/avatar, so the current <img> / initial fallback stays.
• Other hand-rolled menus (e.g. SplitButton).