Add 'Exit Desktop Mode' affordance inside the shell (OS Settings / user menu)

[epeicher]

Summary

The only way to leave Desktop Mode from inside the shell is the admin-bar toggle "Switch to Classic Admin" at includes/admin-bar.php:39-56, which sits among other secondary admin-bar items (Fullscreen, Bug Report, Ask AI, Arrange) and is easy to miss for users who aren't WP-admin-bar-fluent.

There is no exit option in:

• The dock — src/dock.ts has no user menu or exit entry.
• OS Settings — none of src/settings/sections/{help,extended,features}.ts offer a disable option.
• A user/account menu — no such menu exists in the shell. wpd-avatar is purely presentational.

Both automated test runs flagged this independently as "no way to exit Desktop Mode from within the shell."

Suggested fix

Add a clearly-labeled "Switch to Classic Admin" / "Exit Desktop Mode" entry in at least one of:

1. OS Settings → Appearance (most discoverable; matches macOS-style "exit fullscreen" in System Settings). Probably the cleanest place.
2. A user menu off the avatar in the shell (would also unblock Logout, Profile, etc. as a future surface). The avatar component is at wpd-avatar.
3. (Already exists) Admin-bar toggle — keep it.

OS Settings is the lowest-cost win and the most discoverable for users who think to look in "Settings" first.

Severity

Medium. Polish on an existing affordance, not a hard blocker — but discoverability matters for trust.