nix/hive-gateway: serve /matrix/config.json with defaultHomeserver = hive domain (#736)

nix/modules/hive-gateway.nix

@@ -275,6 +275,36 @@ in
                       try_files $uri $uri/ $matrix_spa_target =404;
                     '';
                   };
+                  # FluffyChat fetches `/matrix/config.json` directly on
+                  # boot for its own branding + default-homeserver
+                  # bootstrap, BEFORE asking the user to pick a server
+                  # (#736). The upstream `pkgs.fluffychat-web` dist
+                  # ships without one, so the fetch 404s and the user
+                  # sees the empty "enter homeserver" prompt. Serve a
+                  # minimal config that pre-fills `defaultHomeserver`
+                  # with the operator's hive domain — the matrix
+                  # client then runs `.well-known/matrix/client` against
+                  # that domain (already served by the
+                  # `= /.well-known/matrix/client` block below) and
+                  # discovers the actual tuwunel endpoint.
+                  #
+                  # Only the `defaultHomeserver` field is overridden —
+                  # everything else (branding, audio defaults, etc.)
+                  # falls back to fluffychat's hardcoded defaults so
+                  # we don't pin against upstream config-schema drift.
+                  # No-op when `services.hyperhive.domain` is unset
+                  # (the location block is omitted entirely; the SPA
+                  # then falls back to its empty form, same as before
+                  # #736).
+                } // lib.optionalAttrs (
+                  matrixCfg.enable && matrixCfg.gui.enable && hyperhiveDomain != null
+                ) {
+                  "= /matrix/config.json" = {
+                    extraConfig = ''
+                      default_type application/json;
+                      return 200 '{"defaultHomeserver":"${hyperhiveDomain}"}';
+                    '';
+                  };
                 }
                 //
                   # `.well-known/matrix/*` auto-discovery (#660): when