fix: render raw activity state in inspector

[whoisasx]

Summary

• preserve backend session.activity in the frontend workspace model
• map session.activity through useWorkspaceQuery
• render the Session Inspector Activity section from raw activity.state and activity.lastActivityAt
• remove misleading PR/worktree synthetic rows from the lifecycle Activity section

Closes #2277

Tests

• npm --prefix frontend test -- --run src/renderer/components/SessionInspector.test.tsx src/renderer/hooks/useWorkspaceQuery.test.tsx src/renderer/types/workspace.test.ts
• npm --prefix frontend run typecheck
• git diff --check