fix(20): IN-01 helper text when display name is empty

Render an explanatory helper line when displayName is empty and no
other error is showing, so admins editing an OIDC-provisioned member
with a null displayName understand why Save is disabled rather than
seeing a greyed-out button with no explanation.

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

apps/pwa/src/components/MemberEditorSheet.tsx

@@ -576,6 +576,12 @@ export function MemberEditorSheet({
                 </button>
               </div>
 
+              {/* IN-01: helper text when display name is empty (e.g. null-displayName
+                  OIDC member) so the admin knows why Save is disabled */}
+              {displayName.trim().length === 0 && !profileError && (
+                <p style={helperTextStyle}>Enter a display name to enable Save.</p>
+              )}
+
               {/* Last-admin guard inline error */}
               {profileError && (
                 <div id="profile-error" style={inlineErrorStyle}>