feat(frontend): 2-column layout for EngagementFormPage in edit mode

In edit mode with canEditEngagements, wraps [form | C2ConfigCard] in a
lg:grid-cols-2 responsive grid with items-start alignment. Stacks to
single column on screens narrower than lg. In create mode, retains the
existing max-w-2xl single-column layout. No logic changes.

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

frontend/src/pages/EngagementFormPage.tsx

@@ -124,7 +124,7 @@ export function EngagementFormPage(): JSX.Element {
   const submitting = createMutation.isPending || patchMutation.isPending;
 
   return (
-    <div className="flex flex-col gap-xl max-w-2xl">
+    <div className="flex flex-col gap-xl">
       <header>
         <h1 className="text-[32px] font-medium leading-none">
           {editing ? 'Edit engagement' : 'New engagement'}
@@ -136,6 +136,13 @@ export function EngagementFormPage(): JSX.Element {
         </p>
       </header>
 
+      <div
+        className={
+          editing && canEditEngagements
+            ? 'grid grid-cols-1 lg:grid-cols-2 gap-xl items-start'
+            : 'max-w-2xl'
+        }
+      >
         <form onSubmit={onSubmit} noValidate className="card-product flex flex-col gap-md">
           <FormField label="Name" htmlFor="eng-name" required error={errors.name}>
             <TextInput
@@ -222,5 +229,6 @@ export function EngagementFormPage(): JSX.Element {
           <C2ConfigCard engagementId={numericId} />
         )}
       </div>
+    </div>
   );
 }