feat: add storage info to settings page and improve NFS export handling

Backend:
- Add source/used_by/available/error fields to diskUsage in system status
- collectDiskUsage() now reads /proc/mounts, samba shares and NFS exports from DB
- Paths are deduplicated; shared paths list all shares/exports using them
- syscall.Statfs errors surface as available=false with user-facing error
- collectServiceStatus made a method of Server (receiver consistency)

Frontend:
- Settings page now shows two cards: mount points and shared resources
- Each path shows source badge (Sistema/Mount/SMB/NFS), used_by chips, progress bar
- Unavailable paths show amber warning instead of progress bar
- DiskUsage interface updated with new fields
- NFSExport interface updated with structured fields (fsid, async, etc)
- NFS page updated to use new export fields

internal/modules/nfs/fsid.go

@@ -0,0 +1,32 @@
+package nfs
+
+import (
+	"crypto/rand"
+	"encoding/binary"
+	"fmt"
+)
+
+func generateRandomFSID() (uint32, error) {
+	var b [4]byte
+	if _, err := rand.Read(b[:]); err != nil {
+		return 0, fmt.Errorf("crypto/rand read: %w", err)
+	}
+	n := binary.BigEndian.Uint32(b[:])
+	if n == 0 {
+		n = 1
+	}
+	return n, nil
+}
+
+func UniqueFSID(used map[uint32]struct{}) (uint32, error) {
+	for i := 0; i < 64; i++ {
+		n, err := generateRandomFSID()
+		if err != nil {
+			return 0, err
+		}
+		if _, dup := used[n]; !dup {
+			return n, nil
+		}
+	}
+	return 0, fmt.Errorf("fsid space exhausted after 64 attempts")
+}