feat(workspace): add lifecycle switcher

[decode2]

Linked issue

Closes #60

• Approved issue linked.

Pull request type

• New feature (type:feature)

Summary

• Adds minimal create/select/close/restart/recover workspace controls.
• Preserves workspace/session/UI-tab identity boundaries and defers terminal adoption.

Test plan

• Focused frontend suites: 13/13.
• Lint, typecheck, and build passed.
• No direct pty_spawn or terminal adoption.

Chained pull request context

• Previous PR: feat(workspace): add typed desktop client #59
• Next PR: PR12c — stable backend-session adoption
• Review this PR first: action sequencing and identity boundaries
• Intentionally out of scope: terminal adoption and release work

Contributor checklist

• Duplicate search and approved issue
• Exactly one type label
• Reproducible evidence
• Conventional commit; no AI attribution