Get real mailbox size from dovecot

- Add mail.GetQuota() using doveadm quota get
- Fix UserWithQuota response struct
- Fix frontend NaN display with Unknown fallback
- Update field names to camelCase (usedQuota)

backend/internal/api/handlers/users.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/gin-gonic/gin"
 	"github.com/imc-vibe/backend/internal/db"
+	"github.com/imc-vibe/backend/internal/mail"
 )
 
 type UserHandler struct {
@@ -30,6 +31,13 @@ type UpdateUserRequest struct {
 	Quota    int64  `json:"quota,omitempty"`
 }
 
+type UserWithQuota struct {
+	ID        uint   `json:"id"`
+	Email     string `json:"email"`
+	Quota     int64  `json:"quota"`
+	UsedQuota *int64 `json:"usedQuota"`
+}
+
 func (h *UserHandler) List(c *gin.Context) {
 	domainName := c.Param("name")
 	if domainName == "" {
@@ -61,7 +69,22 @@ func (h *UserHandler) List(c *gin.Context) {
 		return
 	}
 
-	Success(c, users)
+	result := make([]UserWithQuota, len(users))
+	for i, user := range users {
+		result[i] = UserWithQuota{
+			ID:    user.ID,
+			Email: user.Email,
+			Quota: user.Quota,
+		}
+
+		quota, err := mail.GetQuota(user.Email)
+		if err == nil && quota != nil {
+			result[i].Quota = quota.Limit
+			result[i].UsedQuota = &quota.Used
+		}
+	}
+
+	Success(c, result)
 }
 
 func (h *UserHandler) Get(c *gin.Context) {