Collab Phase 2: Realtime Client and Presence UI (No Save Behavior Change Yet)

## Summary
Implement websocket client integration and collaborator presence UI without changing persistence behavior yet.

## Scope
- Add websocket client service with reconnect and token refresh handling.
- Join project and page rooms for active editing context.
- Send and render presence/cursor/selection events.
- Keep current save endpoints unchanged in this phase.
- Gate by feature flag.

## Acceptance Criteria
- Collaborator presence appears in active page.
- Cursor updates are visible with acceptable latency.
- Existing save behavior remains unchanged when feature flag on.
- Full disable when feature flag off.

## Dependency
- Depends on TPEN-services websocket skeleton issue.