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

nix/host-modules/hive-gateway/vhosts.nix

@@ -46,8 +46,7 @@ let
   # by default; legacy deep-link shim during the fluffychat sub-domain move).
   # See `docs/networking/gateway.md`.
   matrixRedirectLocations =
-    lib.optionalAttrs
-      (matrixDeployCfg.enable && matrixDeployCfg.gui.enable && matrixCfg.gatewayHost != null)
+    lib.optionalAttrs (matrixDeployCfg.enable && matrixCfg.gatewayHost != null)
       (
         let
           target = "${publicScheme}://${matrixCfg.gatewayHost}${publicPortSuffix}";