Fix Organization Management dialog type errors

- Fix JSON key mismatch in Flutter models: change 'User' to 'user' in Member and JoinRequest fromJson
- Update backend userInfo to send displayName as *string (null when empty)
- Add json tags to database User struct for consistent lowercase keys
- Handle displayName nullability in backend handlers

b0esche_cloud/lib/models/organization.dart

@@ -21,7 +21,7 @@ class Member {
       orgId: json['OrgID'] ?? json['orgId'],
       role: json['Role'] ?? json['role'],
       createdAt: DateTime.parse(json['CreatedAt'] ?? json['createdAt']),
-      user: User.fromJson(json['User'] ?? json),
+      user: User.fromJson(json['user']),
     );
   }
 }
@@ -90,7 +90,7 @@ class JoinRequest {
       inviteToken: json['InviteToken'] ?? json['inviteToken'],
       requestedAt: DateTime.parse(json['RequestedAt'] ?? json['requestedAt']),
       status: json['Status'] ?? json['status'],
-      user: User.fromJson(json['User'] ?? json),
+      user: User.fromJson(json['user']),
     );
   }
 }

go_cloud/internal/database/db.go

@@ -63,13 +63,13 @@ func (sa StringArray) Value() (driver.Value, error) {
 }
 
 type User struct {
-	ID           uuid.UUID
-	Email        string
-	Username     string
-	DisplayName  string
-	PasswordHash *string
-	CreatedAt    time.Time
-	LastLoginAt  *time.Time
+	ID           uuid.UUID `json:"id"`
+	Email        string    `json:"email"`
+	Username     string    `json:"username"`
+	DisplayName  string    `json:"displayName"`
+	PasswordHash *string   `json:"-"`
+	CreatedAt    time.Time `json:"createdAt"`
+	LastLoginAt  *time.Time `json:"lastLoginAt"`
 }
 
 type Credential struct {

go_cloud/internal/http/routes.go

@@ -897,10 +897,10 @@ type memberResponse struct {
 }
 
 type userInfo struct {
-	ID          string `json:"id"`
-	Username    string `json:"username"`
-	DisplayName string `json:"displayName"`
-	Email       string `json:"email"`
+	ID          string  `json:"id"`
+	Username    string  `json:"username"`
+	DisplayName *string `json:"displayName"`
+	Email       string  `json:"email"`
 }
 
 func listMembersHandler(w http.ResponseWriter, r *http.Request, db *database.DB) {
@@ -924,7 +924,12 @@ func listMembersHandler(w http.ResponseWriter, r *http.Request, db *database.DB)
 			User: userInfo{
 				ID:          m.User.ID.String(),
 				Username:    m.User.Username,
-				DisplayName: m.User.DisplayName,
+				DisplayName: func() *string {
+					if m.User.DisplayName == "" {
+						return nil
+					}
+					return &m.User.DisplayName
+				}(),
 				Email:       m.User.Email,
 			},
 		})
@@ -1191,7 +1196,12 @@ func listJoinRequestsHandler(w http.ResponseWriter, r *http.Request, db *databas
 			User: userInfo{
 				ID:          req.User.ID.String(),
 				Username:    req.User.Username,
-				DisplayName: req.User.DisplayName,
+				DisplayName: func() *string {
+					if req.User.DisplayName == "" {
+						return nil
+					}
+					return &req.User.DisplayName
+				}(),
 				Email:       req.User.Email,
 			},
 		})