Feature: Persist Workflow Studio Shell Layout (splitters, panels, toggles)

### Why
Persist splitter positions, shell panel widths, visibility state (collapsed/expanded), and active shell tabs in Workflow Studio. Save geometry to localStorage (persistent across sessions) and session toggles (collapsed/expanded, tab) to sessionStorage (per tab session), via new persistence service and using unified naming/scoping conventions.

### Acceptance Criteria
- Component state saved on resize/mutation events (debounced)
- Local storage for geometry, session storage for toggles
- Restored state on Studio load and on component re-render
- Easily supports future server-profile provider
- Reset affordance (restore factory defaults)
- Fully integrated end-to-end: from resize drag to storage to restore

### Deliverables
- PR for Workflow Studio shell
- Example usage of the API/hook
- Unit/E2E test or manual validation plan