Add support for manager v4 feature flag and client UI capability:
- MANAGER_SUPPORTS_V4: Server-side flag for v4 manager support
- supports_manager_v4_ui: Client-side flag for v4 UI support
These flags enable proper capability negotiation between frontend and
backend for manager UI selection (legacy vs v4).
Also fix TypeScript errors by adding @types/lodash.
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Co-Authored-By: Claude <noreply@anthropic.com>