Retheme UI from blue to neutral zinc backgrounds with indigo accents

Removes the blue tint from all dark-mode surfaces by switching CSS
variables to zinc-based neutrals, and replaces decorative blue classes
with indigo across buttons, focus rings, tabs, and links. Semantic blue
(severity badges, status badges, role badges, timeline markers) is
preserved.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/src/pages/Login.tsx

@@ -30,7 +30,7 @@ export default function Login() {
   };
 
   const inputClass =
-    'w-full bg-gray-800 border border-gray-700 text-gray-100 placeholder-gray-500 rounded-lg px-3 py-2 text-sm focus:outline-none focus:ring-2 focus:ring-blue-500 focus:border-transparent';
+    'w-full bg-gray-800 border border-gray-700 text-gray-100 placeholder-gray-500 rounded-lg px-3 py-2 text-sm focus:outline-none focus:ring-2 focus:ring-indigo-500 focus:border-transparent';
 
   return (
     <div className="min-h-screen bg-gray-950 flex items-center justify-center px-4">
@@ -70,7 +70,7 @@ export default function Login() {
           <button
             type="submit"
             disabled={isSubmitting}
-            className="w-full bg-blue-600 text-white py-2 rounded-lg text-sm font-medium hover:bg-blue-700 disabled:opacity-50 transition-colors"
+            className="w-full bg-indigo-600 text-white py-2 rounded-lg text-sm font-medium hover:bg-indigo-700 disabled:opacity-50 transition-colors"
           >
             {isSubmitting ? 'Signing in...' : 'Sign in'}
           </button>