feat: add announcements loading state skeleton

[AliMahmoudDev]

What this PR does

Adds a skeleton loading component for the announcements section.

Changes

• Created AnnouncementSkeleton component with configurable count
• Matches the layout of actual announcement items
• Pulse animation for loading feedback
• Dark mode support
• Accessible with role="status" and sr-only text

Closes #27

Usage

{loading ? <AnnouncementSkeleton count={3} /> : <AnnouncementList announcements={data} />}