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

docs/integrations/matrix.md

@@ -8,7 +8,7 @@ homeserver, on one host. Two namespaces configure it:
 - `services.hyperhive.swarm.matrix.*` — what the homeserver **is**, as
   every hive sees it: `serverName`, `gatewayHost`, ports, `allowEncryption`.
 - `services.hyperhive.deploy.matrix.*` — what the host running it decides:
-  `enable`, `gui.enable`, `openFirewall`, `trustedServers`,
+  `enable`, `gui.package`, `openFirewall`, `trustedServers`,
   `maxRequestSize`, `sso.clientSecretFile`.
 
 Vhost routing lives in [`gateway.md`](../networking/gateway.md).