swarm: remove swarm.ui.domain, serve the UI on the swarm domain

The UI and the swarm-controller it fronts are one service to a reader and to a
certificate, so a separate option only ever created two spellings of one
address — and the controller's own public URL was built from one of them while
an allow-list entry named the other.

Removed rather than aliased. A config still setting it was pinning a name the
controller never used; an alias would preserve exactly that mismatch, so
`mkRemovedOptionModule` says so and fails the build.

⚠️ The option was TOTAL and `swarm.domain` is NULLABLE. That is not incidental:
`ui.domain` fell back to `swarm.invalid` so hive-network.nix's required-domain
assertion is what an operator sees, rather than a coercion error from a
consumer. Each site keeps that property its own way — swarm-ui binds a local
total (the idiom swarm-otel.nix already uses as `domainBase`), and the three
sites already gated on `deploy.swarm-ui.enable` gain `&& domain != null`, which
is stricter than a placeholder and puts no invented name into rendered config.

One behaviour change worth naming: `swarm.nix`'s certificate SAN list used to
receive the placeholder `swarm.invalid` on a null domain and now receives
nothing. `lib.optional` would otherwise have put a literal `null` in it.

Verified with `checks.module-eval` rather than `nix-instantiate --parse`: parse
proves syntax, and every failure this change could plausibly cause — a dead
binding, a renamed reference — is an evaluation error.

nix/host-modules/hive-forge/default.nix

@@ -817,15 +817,13 @@ in
               # Not gated on `deploy.swarm-ui.enable`: in a spread deployment the
               # forge and the swarm host are different machines, so that
               # per-host flag is false exactly where this entry is needed.
-              # `swarm.ui.domain` is the option the controller builds its public
-              # URL from; it is slated to collapse into `swarm.domain`.
               webhook.ALLOWED_HOST_LIST = lib.concatStringsSep "," (
                 lib.unique (
                   [
                     "external"
                     hyperhiveDomain
                   ]
-                  ++ lib.optional (swarmDomain != null) config.services.hyperhive.swarm.ui.domain
+                  ++ lib.optional (swarmDomain != null) swarmDomain
                 )
               );
               log.LEVEL = "Warn";