Custom Domains (DNS verification + domain links)

First public release of custom domains: users can add subdomains, copy required DNS records, verify ownership, and create short links under their domain.

From the dashboard users can add a subdomain (e.g. short.example.com). The UI shows the DNS record (type, name, value, TTL) required for verification and provides copy buttons for each value.

Verification is performed via an explicit "Verify Domain" action which calls the verify endpoint; successful verification updates the domain to "Verified" and unlocks domain-scoped actions.

Once verified, admins can create short links under the domain, view per-domain summaries (active links, total clicks, last used), and adjust domain settings such as SSL and protection toggles.

Add domain management UI (add/delete domain, show verification DNS record: type/name/value/ttl)

Verify domain endpoint and client flow — retries & explicit "Verify Domain" action

Domain-scoped links API + UI: list domain links, create short path, delete domain link

Per-domain summary cards (active links, total clicks, last used) and tabs for Overview/Links/Settings