Sprints screen, full search results page, and an archive/trash page

Part of PAR-357.

Problem

• Sprints have no screen. They exist as a feature: there's a sprint property on issues and in new-issue, sprint settings, and analytics references. You can't see the current sprint, its progress, scope changes or what rolls over.
• Search lives only in the ⌘K palette (a handful of results). There's no results page with filters, highlighted matches, or comment and description hits.
• Archived issues are hidden behind an "Archived issues" toggle on the list. There's no archive or trash view with restore and permanent delete. PAR-390 adds undo; this task gives a home to what's already archived.

Do

• Sprints (per team, when the feature is on):
  • a current-sprint header with dates, a progress bar (scope / started / done) and a small burn-up
  • an issue list grouped by status
  • upcoming and past sprints
  • "Move unfinished to next sprint" when closing
  • No estimates or points (decision 2026-10-17): count issues only.
• Search: / or ⌘K → "See all results" opens /search?q=. Results are grouped by issues, projects and comments, with highlighted snippets, filter chips (team, status, assignee, date), and keyboard navigation and peek. Uses the semantic search work (PAR-228) for "similar" matches.
• Archive: a workspace archive page listing archived issues (and later projects) with who and when, restore, and delete permanently (admins, type-to-confirm). Auto-archive settings can live in workspace settings.