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/admin/Users.tsx

@@ -174,7 +174,7 @@ export default function AdminUsers() {
   };
 
   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';
   const labelClass = 'block text-sm font-medium text-gray-300 mb-1';
 
   return (
@@ -183,7 +183,7 @@ export default function AdminUsers() {
       action={
         <button
           onClick={openAdd}
-          className="flex items-center gap-2 bg-blue-600 text-white px-4 py-2 rounded-lg text-sm hover:bg-blue-700 transition-colors"
+          className="flex items-center gap-2 bg-indigo-600 text-white px-4 py-2 rounded-lg text-sm hover:bg-indigo-700 transition-colors"
         >
           <Plus size={14} />
           Add User
@@ -281,7 +281,7 @@ export default function AdminUsers() {
               </div>
               <button
                 onClick={closeModal}
-                className="w-full bg-blue-600 text-white py-2 rounded-lg text-sm hover:bg-blue-700 transition-colors"
+                className="w-full bg-indigo-600 text-white py-2 rounded-lg text-sm hover:bg-indigo-700 transition-colors"
               >
                 Done
               </button>
@@ -371,7 +371,7 @@ export default function AdminUsers() {
                 <button
                   type="submit"
                   disabled={submitting}
-                  className="px-4 py-2 text-sm bg-blue-600 text-white rounded-lg hover:bg-blue-700 disabled:opacity-50 transition-colors"
+                  className="px-4 py-2 text-sm bg-indigo-600 text-white rounded-lg hover:bg-indigo-700 disabled:opacity-50 transition-colors"
                 >
                   {submitting ? 'Saving...' : modal === 'add' ? 'Create User' : 'Save Changes'}
                 </button>