fix(headless-login): show full parsed jwks key values

- return the full RSA n value in parsedKeys responses
- render parsed key fields with labels and multiline key material in DevFront
- lock the behavior with backend and Playwright regression tests

backend/internal/domain/headless_jwks_cache.go

@@ -3,11 +3,11 @@ package domain
 import "time"
 
 type HeadlessJWKSParsedKey struct {
-	Kid      string `json:"kid,omitempty"`
-	Kty      string `json:"kty,omitempty"`
-	Use      string `json:"use,omitempty"`
-	Alg      string `json:"alg,omitempty"`
-	NPreview string `json:"nPreview,omitempty"`
+	Kid string `json:"kid,omitempty"`
+	Kty string `json:"kty,omitempty"`
+	Use string `json:"use,omitempty"`
+	Alg string `json:"alg,omitempty"`
+	N   string `json:"n,omitempty"`
 }
 
 // HeadlessJWKSCacheState는 headless login용 JWKS 캐시 상태와 최근 동기화 결과를 나타냅니다.