feat(ui): add return-to-desktop sidebar action

[Che-Zhu]

Summary

• add an always-visible return-to-Sealos-Desktop action above the sidebar Upgrade control
• request closeDesktopApp with the canonical system-brain app key
• add sidebar contract tests for the event payload, placement, and intentionally unconditional rendering

Why

Sealos Desktop suppresses the normal window title bar for system-brain, so Brain needs its own explicit control to close the app iframe and return users to the desktop.

Validation

• bun test apps/ui/src/components/app-sidebar.test.ts apps/ui/src/components/app-sidebar-upgrade.test.ts — 7 passed
• bunx ultracite check apps/ui/src/components/app-sidebar.tsx apps/ui/src/components/app-sidebar.test.ts
• bun typecheck
• git diff --check upstream/main...HEAD

Known repository check

The full bun check remains blocked by pre-existing formatting findings in apps/ui/drizzle/meta/0004_snapshot.json and apps/ui/drizzle/meta/_journal.json; this PR does not modify either file.