Adds notification center to the header

[Mbeaulne]

Description

Adds a read-only Notices popover to the top-right of both application headers. It shows the active notice count and lists the same active announcements used by the homepage banners, preserving host order and rendering their Markdown content.

This intentionally does not add read/unread tracking, separate dismissal state, polling, or notification subscriptions.

Related Pull Requests

• Adds markdown support for banners #2677

Type of Change

• New feature

Test Instructions

1. Supply several announcements through window.__TANGLE_ANNOUNCEMENTS__.
2. Confirm the notice count appears in both header versions.
3. Open the popover and confirm active notices appear in host order with Markdown links.
4. Confirm expired notices are omitted and an empty source shows No active notices.
5. Confirm dismissing a homepage banner does not remove it from the read-only notice list.

Automated coverage:

pnpm exec vitest run src/components/shared/NoticeCenter.test.tsx
pnpm run typecheck

[github-actions]

🎩 Preview

A preview build has been created at: 08-28-adds_notification_center_to_the_header/dbb7dc1

[Mbeaulne]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Adds a bunch of development test announcments #2679
• Adds notification center to the header #2678 👈 (View in Graphite)
• Adds markdown support for banners #2677
• master

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