swarm-ui: build the forge link from swarm.forge.domain

The swarm-controller builds the Forge quick link from
services.hyperhive.swarm.forge.domain, replacing hive-forge/default.nix's
per-host entry, so all seven swarm-service links come from swarm-level
options.

Also drops the remaining references to the removed matrix GUI switch:
the HiveUrls / Urls / hive_urls docs, the hivectl.md `open` note and
the gateway.md vhost-map rows, which name `gatewayHost` instead. The
grafana, victoriametrics and victorialogs modules' comments no longer
mention a quick-link they do not define.

Refs #4885

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

@@ -252,16 +252,6 @@ in
     # The forge container resolves the rest of the hive through dnsmasq.
     services.hyperhive.gateway.dns.enable = lib.mkDefault true;
 
-    # This swarm-ui quick-links entry. See
-    # `services.hyperhive.swarm.controller.links`'s description.
-    services.hyperhive.swarm.controller.links = [
-      {
-        label = "Forge";
-        icon = "⚒";
-        url = "https://${cfg.domain}/";
-      }
-    ];
-
     # The metrics endpoint, declared once: the collector both scrapes this
     # URL and derives from it the audience its token is minted for.
     #