feat: high-contrast theme (#29805)

.changeset/smooth-planes-cough.md

...ient/components/GenericUpsellModal/GenericUpsellModal.tsx

apps/meteor/client/providers/UserProvider/UserProvider.tsx

apps/meteor/client/sidebar/header/hooks/useAccountItems.tsx

apps/meteor/client/sidebar/header/hooks/useThemeItems.tsx

apps/meteor/client/sidebar/header/hooks/useUserMenu.tsx

...nt/views/account/preferences/PreferencesGlobalSection.tsx

apps/meteor/client/views/account/routes.tsx

apps/meteor/client/views/account/sidebarItems.tsx

...r/client/views/account/themes/HighContrastUpsellModal.tsx

apps/meteor/client/views/account/themes/ThemePage.tsx

apps/meteor/client/views/account/themes/themeItems.ts

apps/meteor/packages/rocketchat-i18n/i18n/en.i18n.json

apps/meteor/public/images/high-contrast-upsell-modal.png

apps/meteor/server/methods/saveUserPreferences.ts

ee/packages/ui-theming/src/PaletteStyleTag.tsx

ee/packages/ui-theming/src/hooks/useThemeMode.ts

ee/packages/ui-theming/src/palette.ts

ee/packages/ui-theming/src/paletteHighContrast.ts

ee/packages/ui-theming/src/types/themes.ts