feat: CRUD digital contacts

[pskillen]

Problem

Digital contacts can only be viewed, not managed. They are distinct from DMR talk groups.

Intended outcome

Full create / read / update / delete for digital contacts against the internal data models.

• Add, edit, and delete digital contacts.
• Model the distinction clearly: under the hood a TG = a group call to a DMR ID, a contact = a private call to a DMR ID.
• Keep references consistent where channels/TG lists reference contacts.

Affected tool(s)

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

Notes

• Depends on internal data models (genericise-import) and LocalStorage persistence.
• Related to (but separate from) CRUD DMR talk groups and TG lists.