design: two-color palette — rename all tokens to --cream / --blue

Replace ochre-clay, carbon-black, burnt-oxide, slate-indigo with clean
two-color system: --cream (#f4f0e8) and --blue (#041cf3). Update every
component, utility class, and test assertion.

src/widgets/Navigation/ui/SidebarNav.tsx

@@ -37,7 +37,7 @@ export function SidebarNav({ items }: Props) {
   }
 
   return (
-    <nav className="fixed top-0 left-0 h-screen w-full lg:w-1/3 bg-ochre-clay brutal-border-right hidden lg:block overflow-y-auto z-50">
+    <nav className="fixed top-0 left-0 h-screen w-full lg:w-1/3 bg-cream brutal-border-right hidden lg:block overflow-y-auto z-50">
       <div className="px-8 py-12 space-y-2">
         <div className="mb-12">
           <h2>Index</h2>
@@ -51,9 +51,9 @@ export function SidebarNav({ items }: Props) {
             key={item.id}
             href={`/${item.id}`}
             className={cn(
-              'block w-full text-left brutal-border bg-ochre-clay px-6 py-4 transition-all duration-300',
+              'block w-full text-left brutal-border bg-cream px-6 py-4 transition-all duration-300',
               isActive(item)
-                ? 'shadow-[12px_12px_0_var(--carbon-black)] opacity-100 translate-x-0'
+                ? 'shadow-[12px_12px_0_var(--blue)] opacity-100 translate-x-0'
                 : 'opacity-40 shadow-none hover:opacity-60',
             )}
           >