[FEAT] Implement real archive views

[martian56]

Summary
The Archives route exists, but it is only a placeholder and does not show archived projects or work items.

Where I saw it

• ui/src/pages/ArchivesPage.tsx renders static placeholder text.
• api/internal/router/router.go does not expose archive-specific endpoints for projects or issues.
• Several models have archived_at fields, but there is no complete archive browsing flow.

Why this matters
The sidebar exposes Archives as a real destination. Users who click it expect to recover or review archived work, but the page currently cannot do anything useful.

Suggested fix

• Add API support for listing archived projects and work items by workspace.
• Add restore or unarchive actions if archived data is meant to be recoverable.
• Build the Archives page with empty, loading, error, and populated states.
• Hide the route until it is functional if this is not in scope yet.

Acceptance criteria

• Archives page lists archived projects and work items for the current workspace.
• Users can open archived items from the list.
• Users with the right role can restore archived items.
• The page no longer shows placeholder copy.

Checked on
main at f8e5946