feat(admin): improve identity labels and navigation

[konglyyy]

Summary

• display workspace creator names or email addresses before falling back to immutable user IDs
• display workspace names in admin audit objects while retaining immutable workspace IDs for filtering and details
• revise navigation to keep Overview standalone, group Workspaces, Users, and Platform Settings under Admin Management, and place Admin Audit under Governance
• preserve existing routes, icons, permissions, active navigation, and accessible category semantics
• update requirements, design, contract documentation, and UI alignment coverage

Cross-repository change

This is the consumer half of the admin readable-identities change.

Merge order:

1. control-plane (producer PR)
2. platform-admin-console (this PR)

Related PR: acornops/control-plane#20

Validation

• npm run validate: passed
• 95 tests passed
• lint, contract, requirements, harness, build, and route smoke checks passed
• workspace platform contract check passed
• desktop and compact navigation visually verified for ordering, active state, spacing, and overflow
• rebuilt local stack verified healthy

Documentation impact

• updated current requirements, requirements baseline, design notes, contract documentation, and the completed execution plan

Compatibility and residual risk

Routes and authorization behavior are unchanged. Readable labels fall back to immutable IDs when the additive control-plane fields are absent, so mixed-version deployments remain usable.