feat(workspace): add typed desktop client

[decode2]

Linked issue

Closes#58

• The linked issue has the status:approved label.

Pull request type

• New feature (type:feature)

Summary

• Adds a typed frontend client for the seven workspace commands.
• Separates WorkspaceId, backend SessionId, and UI TabId while exposing deterministic capability/loading/error state.

Changes

Area	Change
workspaceClient.ts	Typed command client, branded identities, state reducer.
workspaceClient.test.ts	Command mapping, identity, state, and no-pty-spawn contracts.

Test plan

• Focused frontend suite: 23/23.
• Lint, typecheck, and production build passed.
• Verified no rendered UI or terminal spawning behavior changed.

Chained pull request context

• Previous PR: feat(workspace): expose lifecycle command authority #57
• Next PR: PR12b — minimal workspace switcher/actions
• Review this PR first: identity boundaries and command/state contracts
• Intentionally out of scope: rendered UI, terminal session adoption, and release work

Contributor checklist

• Duplicate search completed.
• Linked approved issue.
• Exactly one PR type/label.
• Reproducible test evidence included.
• Documentation deferred to release slice.
• Conventional commit; no AI attribution.