feat: persist state in browser LocalStorage

[pskillen]

Problem

State is currently held in memory only (with the small exception of the Mapbox API key). Once users can write/modify/export CPS data, losing everything on refresh is unacceptable.

Intended outcome

Persist the working CPS state (internal data models) to browser LocalStorage so it survives reloads.

• Save/restore the internal models on load.
• Sensible handling of clearing/replacing state on a new import.
• Keep within LocalStorage size constraints; document limits.

Affected tool(s)

• Shared state/storage layer consumed by tools/.

Notes

• Privacy: never commit operator codeplugs; storage is browser-local only. Mapbox token stays in LocalStorage only, never in the repo.
• Depends on the internal data models (genericise-import) and pairs with the CRUD tickets.