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

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 {