feat(domains): support apex domains and non-.com TLDs

Loosen the custom-domain regex to accept apex domains (1+ labels) in
addition to subdomains; the TLD already allowed any letters. Suppress
the CNAME record for apex hostnames since a CNAME is invalid at the zone
apex, and surface only the A record there. Drop the frontend apex
rejection and the misleading ".com" input suffix, and render the
domain-input prefix/suffix addons only when set.

apps/api/package.json

@@ -67,6 +67,7 @@
     "katex": "^0.16.10",
     "marked": "^7.0.3",
     "nanoid": "^5.1.5",
+    "tldts": "^6.1.86",
     "zod": "^4.1.8",
     "zod-openapi": "^5.4.1"
   },