move swaggerUiTheme nix option out of c0re's namespace

services.hyperhive.c0re.swaggerUiTheme -> services.hyperhive.gateway.swaggerUiTheme.
hive-c0re has no relationship to this option (it's consumed entirely
by the gateway's nginx config) since the previous two commits moved
the theme files out of hive-c0re/ and gateway to hosting the whole
dist itself — the option shouldn't live in c0re's namespace either.

Pure option move: default wiring in flake.nix now sets
services.hyperhive.gateway.swaggerUiTheme instead of the c0re one;
hive-gateway/default.nix reads it via the module's own already-bound
cfg (services.hyperhive.gateway) instead of reaching into c0re's.

Verified: nix build .#swagger-ui-theme unaffected (same store path,
pure option-plumbing change, no content change); a full nixosSystem
eval of nixosModules.default resolves the option at its new path and
confirms the gateway's /api/docs/ location still aliases the correct
store path.

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

@@ -27,8 +27,9 @@ let
   # Full hyperhive-themed Swagger UI dist — nginx serves this whole
   # tree straight from the store at /api/docs/, no hive-c0re fallback
   # (see vhosts.nix's `swaggerUiLocations` and
-  # nix/packages/swagger-ui-{dist,theme}.nix).
-  swaggerUiTheme = config.services.hyperhive.c0re.swaggerUiTheme;
+  # nix/packages/swagger-ui-{dist,theme}.nix). Own option under this
+  # module (not c0re's) — hive-c0re has no relationship to it.
+  swaggerUiTheme = cfg.swaggerUiTheme;
 
   # Self-signed TLS is the implicit floor: when neither an operator cert
   # (`tls.certDir`) nor ACME (`tls.acme.enable`) is configured, the gateway