fix(web): stabilize invitation error assertions

The invitation error copy also renders as a transient sonner toast, so the
plain text locator resolved to two elements and tripped Playwright strict
mode while the toast was on screen.

Tag the inline error region with data-testid="invitation-error" and assert
against it, keeping the check deterministic under CI parallelism.

web/src/app/invitations/accept/page.tsx

@@ -296,7 +296,7 @@ export default function AcceptInvitationPage() {
             <div className="space-y-4">
               <div className="flex items-start gap-2 rounded-lg border border-destructive/20 bg-destructive/5 p-3 text-sm text-destructive">
                 <AlertCircle className="mt-0.5 size-4 shrink-0" />
-                <span>{errorMessage}</span>
+                <span data-testid="invitation-error">{errorMessage}</span>
               </div>
               <Button
                 variant="outline"
@@ -313,7 +313,7 @@ export default function AcceptInvitationPage() {
               {errorMessage && (
                 <div className="flex items-start gap-2 rounded-lg border border-destructive/20 bg-destructive/5 p-3 text-sm text-destructive">
                   <AlertCircle className="mt-0.5 size-4 shrink-0" />
-                  <span>{errorMessage}</span>
+                  <span data-testid="invitation-error">{errorMessage}</span>
                 </div>
               )}
 

web/tests/e2e/invitations.spec.ts

@@ -27,9 +27,9 @@ test('terminal invitation errors refresh on a new fragment and allow account swi
   });
 
   await page.goto('/invitations/accept#token=used-invitation');
-  await expect(
-    page.getByText('This invitation was already used.'),
-  ).toBeVisible();
+  await expect(page.getByTestId('invitation-error')).toContainText(
+    'This invitation was already used.',
+  );
   await expect
     .poll(() =>
       page.evaluate(() =>
@@ -41,7 +41,9 @@ test('terminal invitation errors refresh on a new fragment and allow account swi
   await page.evaluate(() => {
     window.location.hash = 'token=revoked-invitation';
   });
-  await expect(page.getByText('This invitation was revoked.')).toBeVisible();
+  await expect(page.getByTestId('invitation-error')).toContainText(
+    'This invitation was revoked.',
+  );
 
   await page.getByRole('button', { name: 'Back to sign in' }).click();
   await expect(page).toHaveURL(/\/login$/);
@@ -101,9 +103,11 @@ test('login preserves an explicit invitation email mismatch error', async ({
   await expect(page).toHaveURL(
     /\/invitations\/accept\?error=invitation_email_mismatch$/,
   );
-  await expect(
-    page.getByText('This invitation belongs to a different email address.'),
-  ).toBeVisible();
+  // The same copy is also shown as a transient toast, so target the page's
+  // inline error region to keep the assertion deterministic.
+  await expect(page.getByTestId('invitation-error')).toContainText(
+    'This invitation belongs to a different email address.',
+  );
   await expect(page.getByText('Login successful')).toHaveCount(0);
 });