style: update ui and design of system notification components (what's new, new release notification, help center)

[christian-byrne]

Summary

Migrated help center and release notification components from hardcoded colors to semantic design tokens for automatic light/dark theme support.

Changes

• What: Replaced hardcoded hex/rgb colors with semantic tokens in HelpCenterMenuContent, WhatsNewPopup, and ReleaseNotificationToast components
• Design System: Added --interface-menu-surface and --interface-menu-stroke tokens to style.css for consistent menu theming
• UX: Updated help center menu structure - added "Give Feedback" button, renamed "Help & Feedback" to "Help & Support", switched to Lucide icons (except Discord brand logo), added external-link icons

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