fix(copilot): add username fallback and consistent file name prefix

- Add 'github-user' fallback in WaitForAuthorization when FetchUserInfo
  returns empty Login (fixes malformed 'github-copilot-.json' filenames)
- Standardize Web API file name to 'github-copilot-<user>.json' to match
  CLI path convention (was 'github-<user>.json')

Addresses Gemini Code Assist review comments on PR #291.

internal/api/handlers/management/auth_files.go

@@ -1974,7 +1974,7 @@ func (h *Handler) RequestGitHubToken(c *gin.Context) {
 			Type:        "github-copilot",
 		}
 
-		fileName := fmt.Sprintf("github-%s.json", username)
+		fileName := fmt.Sprintf("github-copilot-%s.json", username)
 		label := userInfo.Email
 		if label == "" {
 			label = username

internal/auth/copilot/copilot_auth.go

@@ -87,9 +87,14 @@ func (c *CopilotAuth) WaitForAuthorization(ctx context.Context, deviceCode *Devi
 		log.Warnf("copilot: failed to fetch user info: %v", err)
 	}
 
+	username := userInfo.Login
+	if username == "" {
+		username = "github-user"
+	}
+
 	return &CopilotAuthBundle{
 		TokenData: tokenData,
-		Username:  userInfo.Login,
+		Username:  username,
 		Email:     userInfo.Email,
 		Name:      userInfo.Name,
 	}, nil