chore(ui): remove code orphaned by the shortcut and plan-action cleanups

The eslint pass in release:prepare caught what the package-scoped checks
did not: Header's handleOpenContextPlan and servicesTabs lost their last
callers with the removed shortcuts, the settings-synced listeners no
longer need the DesktopSettings import, and the store's openContextPlan
action itself went unused once the plan surface was reachable only through
the digit switcher and the rail.

packages/ui/src/contexts/ThemeSystemContext.tsx

@@ -7,7 +7,6 @@ import React, {
 } from 'react';
 import { flushSync } from 'react-dom';
 import type { Theme, ThemeMode } from '@/types/theme';
-import type { DesktopSettings } from '@/lib/desktop';
 import { isDesktopLocalOriginActive, isDesktopShell as detectDesktopShell, isVSCodeRuntime } from '@/lib/desktop';
 import { setDesktopWindowTheme } from '@/lib/desktopNative';
 import { CSSVariableGenerator } from '@/lib/theme/cssGenerator';