matrix: serve the web client unconditionally; link it from the swarm domain

Removes services.hyperhive.deploy.matrix.gui.enable and its
swarm.matrix.gui.enable alias; both are mkRemovedOptionModule stubs. A
host running the homeserver serves fluffychat at gatewayHost's vhost,
and the hive's /matrix/ redirect follows the same condition.

The swarm-controller builds the Matrix quick link from
swarm.matrix.gatewayHost, replacing hive-matrix.nix's per-host entry.
HIVE_MATRIX_PUBLIC_URL is set on every hive with a gatewayHost, so
`hivectl open matrix` resolves off the homeserver's host too.

Drops HIVE_MATRIX_GUI_ENABLED and the dashboard's matrix_gui_enabled
field; nothing in the frontend reads it.

Refs #4885

hivectl/src/open.rs

@@ -30,7 +30,7 @@ pub(crate) async fn open_url(socket: &Path, target: OpenTarget) -> Result<()> {
         ),
         OpenTarget::Matrix => (
             urls.matrix,
-            "the matrix GUI URL needs `services.hyperhive.deploy.matrix.gui.enable = true`",
+            "the matrix GUI URL needs `services.hyperhive.swarm.matrix.gatewayHost` set",
         ),
     };
     let url = url.with_context(|| format!("no URL available for this surface — {hint}"))?;