fix(issues): S4a typecheck — allow null color in updateSystem param

vue-tsc TS2345: System.color is string|null, but updateSystem's data param
typed color as string, so the store's Partial<Pick<System,...>> wasn't
assignable. Widen the param's color to string|null (clearing a color is valid).

Refs plan 825 (S4a).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/api/systems.ts

@@ -31,7 +31,7 @@ export async function updateSystem(
   data: Partial<{
     name: string;
     description: string;
-    color: string;
+    color: string | null;
     status: "active" | "archived";
     order_index: number;
   }>,