fix(headless-login): show parsed jwks n preview

- reproduce the missing n preview with the actual parsedKeys response shape
- read nPreview from DevFront instead of the old n field
- keep the preview text as provided by backend summaries

devfront/tests/devfront-clients-lifecycle.spec.ts

@@ -149,7 +149,8 @@ test.describe("DevFront clients lifecycle", () => {
                 kty: "RSA",
                 use: "sig",
                 alg: "RS256",
-                n: "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789",
+                nPreview:
+                  "voVbHlo_UHkj...Hb8PiTCQ",
               },
             ],
           },
@@ -229,7 +230,7 @@ test.describe("DevFront clients lifecycle", () => {
       await expect(page.getByText(algorithm, { exact: true }).last()).toBeVisible();
     }
     await expect(
-      page.getByText("abcdefghij...0123456789", { exact: true }),
+      page.getByText("voVbHlo_UHkj...Hb8PiTCQ", { exact: true }),
     ).toBeVisible();
     await expect(
       page.getByRole("button", { name: /refresh|새로고침/i }),