dashboard: optional matrix GUI static mount at /matrix (#607 v0)

nix/modules/hive-matrix.nix

@@ -155,6 +155,41 @@ in
         registration tokens.
       '';
     };
+
+    gui = {
+      enable = lib.mkOption {
+        type = lib.types.bool;
+        default = false;
+        description = ''
+          Mount a matrix web client (default `pkgs.fluffychat-web`) as
+          a static sibling of the dashboard at `/matrix/`, served by
+          hive-c0re's own axum router via the `HIVE_MATRIX_GUI_DIR`
+          env var (#607 v0). Off by default; flip to `true` once the
+          operator wants matrix chat one tab away from the dashboard
+          without standing up a separate gateway.
+
+          Same-origin via hive-c0re is the simplest single-host
+          shape; the post-#15 nginx-front re-root (`https://matrix.''${hyperhive.domain}`)
+          is tracked separately in #609. fluffychat-web supports
+          per-login server pick — point it at the in-host tuwunel URL
+          (`http://localhost:8008` by default) the first time.
+        '';
+      };
+
+      package = lib.mkOption {
+        type = lib.types.package;
+        default = pkgs.fluffychat-web;
+        defaultText = lib.literalExpression "pkgs.fluffychat-web";
+        description = ''
+          Static web client dist to serve at `/matrix/`. Defaults to
+          `pkgs.fluffychat-web` — first-class threads support, e2ee
+          + voice + spaces, full feature surface in the flutter→web
+          build. Override to swap for `hydrogen-web` (lightest),
+          `cinny` (no threads), `element-web` (heaviest, full
+          features), or an out-of-tree client dist.
+        '';
+      };
+    };
   };
 
   config = lib.mkIf cfg.enable {