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/mail/quota.go

@@ -0,0 +1,40 @@
+package mail
+
+import (
+	"bufio"
+	"os/exec"
+	"regexp"
+	"strconv"
+	"strings"
+)
+
+type Quota struct {
+	Used  int64 `json:"used"`  // Used storage in bytes
+	Limit int64 `json:"limit"` // Storage limit in bytes (-1 = unlimited)
+}
+
+func GetQuota(email string) (*Quota, error) {
+	cmd := exec.Command("/usr/bin/doveadm", "quota", "get", "-u", email)
+	output, err := cmd.Output()
+	if err != nil {
+		return nil, err
+	}
+
+	return parseDoveadmQuota(output)
+}
+
+var quotaLineRegex = regexp.MustCompile(`STORAGE\s+(\d+)\s+(\d+)`)
+
+func parseDoveadmQuota(output []byte) (*Quota, error) {
+	scanner := bufio.NewScanner(strings.NewReader(string(output)))
+	for scanner.Scan() {
+		line := scanner.Text()
+		matches := quotaLineRegex.FindStringSubmatch(line)
+		if len(matches) == 3 {
+			used, _ := strconv.ParseInt(matches[1], 10, 64)
+			limit, _ := strconv.ParseInt(matches[2], 10, 64)
+			return &Quota{Used: used, Limit: limit}, nil
+		}
+	}
+	return nil, nil
+}