feat: CRUD zones (incl. add/remove channels)

[pskillen]

Problem

Zones can only be viewed, not managed.

Intended outcome

Full create / read / update / delete for zones against the internal data models, including adding and removing member channels.

• Add, edit, and delete zones.
• Manage zone membership (add/remove channels), respecting OpenGD77 limits (e.g. up to 80 channels per zone).
• Member references use case-sensitive channel names; keep them consistent on channel rename/delete.

Affected tool(s)

• Tools under tools/ plus shared model/state code.

Notes

• Depends on internal data models (genericise-import), LocalStorage persistence, and CRUD channels.

[pskillen]

Implementation started on branch 11/paddy/channels-zones-crud. Combined with #11 in one PR (Closes #11, Closes #12).