feat: show announcement banners

[EhabY]

Summary

• Surface active Coder service/announcement banners in a compact status bar item.
• Show popup notifications only for newly seen banner fingerprints per deployment, respecting coder.disableNotifications.
• Add Coder: View Announcements QuickPick for banner details.
• Split banner normalization/copy from the VS Code manager to keep the implementation and tests concise.

Testing

• pnpm typecheck
• pnpm lint
• pnpm format:check
• pnpm test:extension
• pnpm build

Implementation notes

• Status bar always reflects all active banners using a small label: $(megaphone) Coder or $(megaphone) Coder N.
• Popups are delta-only: each fetch compares active banner fingerprints with per-deployment seen fingerprints.
• No dismiss action is needed because closing the popup is enough; the status bar remains the persistent low-noise surface.
• Banner fingerprints use source, message, and background color because the API does not expose stable banner IDs or timestamps.
• The coder.disableNotifications setting suppresses popups, not the passive status bar item.
• Fetch cadence is activation/session change plus a 30-minute authenticated poll and manual refresh before opening the QuickPick.

Generated by Coder Agents on behalf of @EhabY.