feat: luminous golden toast + link support + DELETE endpoint

[JheisonMB]

Changes

Announcements worker

• DELETE /:id — removes entry from both SQLite and KV mirror (auth required)
• Link field — optional parameter on PUT, stored in SQLite and returned in GET/SSE

Global toast

• Luminous golden glow effect with backdrop blur
• Close button (×) + backdrop click to dismiss
• 8s auto-dismiss with smooth fade
• Clickable when announcement has a link → navigates to the URL
• Golden border + title text (#d4a843)