fix(unit-2-auth): Fix all 4 failing unit tests to 29/29 passing

- Fix invitation MSW handler: use 'valid-token' to match test fixtures
- Add data-testid='setup-success' to SetupPage success state
- Change invite email input from readOnly to disabled (matches test assertion)

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

frontend/src/pages/SetupPage.tsx

@@ -63,7 +63,7 @@ export function SetupPage() {
 
     if (successMessage) {
         return (
-            <div className="flex min-h-svh items-center justify-center bg-secondary/40 p-4">
+            <div className="flex min-h-svh items-center justify-center bg-secondary/40 p-4" data-testid="setup-success">
                 <Card className="w-full max-w-sm">
                     <CardContent className="pt-6">
                         <div className="text-center space-y-4">