swarm-ui: apply the operator's stylix theme, same as the dashboard already does

mara: "swarm dash is in my own colors, but tab colors on swarm ui are
the default catpucchin one." Root cause: hive-c0re/theme.nix's stylix
overlay only ever wrote a themed colors.css onto the dashboard/agent
frontend subtrees -- swarm-ui, served from its own separate package,
was never in scope.

Extracts the stylix-detection + colors.css-generation logic (previously
inline in theme.nix) into a shared nix/host-modules/stylix-theme.nix,
imported by both theme.nix and swarm-ui.nix -- one source instead of a
second copy that has to agree by inspection. swarm-ui.nix gains its own
themedPackage overlay (same shape as theme.nix's themedFrontend: copy
the package, overwrite static/colors.css) and serves that instead of
cfg.package directly when stylix is active; a clean passthrough
otherwise.

Verified with a throwaway nixosSystem eval (this repo's own flake
checks do not exercise gateway-module wiring) confirming the unthemed
path resolves cfg.package unchanged.

nix/host-modules/hive-c0re/theme.nix

@@ -7,6 +7,10 @@
 # from the 16 base16 slots (see docs/web-ui/css-vars.md). The guarded
 # access makes this a clean no-op when stylix isn't imported into the
 # host config. Exposed as the read-only `c0re.servedFrontend` option.
+#
+# Detection + CSS-generation itself lives in `../stylix-theme.nix`
+# (shared with `swarm-ui.nix`) — this file is the c0re-specific overlay
+# consumer (which subtrees, which option).
 {
   pkgs,
   lib,
@@ -15,33 +19,8 @@
 }:
 let
   cfg = config.services.hyperhive.c0re;
-  stylixThemeColors =
-    if (config.stylix.enable or false) && ((config.lib.stylix or { }) ? colors) then
-      config.lib.stylix.colors.withHashtag
-    else
-      null;
-  themedColorsCss =
-    c:
-    pkgs.writeText "hyperhive-colors.css" ''
-      :root {
-        --base00: ${c.base00};
-        --base01: ${c.base01};
-        --base02: ${c.base02};
-        --base03: ${c.base03};
-        --base04: ${c.base04};
-        --base05: ${c.base05};
-        --base06: ${c.base06};
-        --base07: ${c.base07};
-        --base08: ${c.base08};
-        --base09: ${c.base09};
-        --base0A: ${c.base0A};
-        --base0B: ${c.base0B};
-        --base0C: ${c.base0C};
-        --base0D: ${c.base0D};
-        --base0E: ${c.base0E};
-        --base0F: ${c.base0F};
-      }
-    '';
+  stylixTheme = import ../stylix-theme.nix { inherit lib config pkgs; };
+  inherit (stylixTheme) stylixThemeColors themedColorsCss;
   # Overlay the generated colors.css onto both dist subtrees. Both the
   # dashboard (served by hive-c0re via HIVE_STATIC_DIR) and the agent UIs
   # (served by the gateway from HIVE_AGENT_FRONTEND_DIR — static files