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/db/models.go

@@ -38,12 +38,25 @@ type SambaShare struct {
 }
 
 type NFSExport struct {
-	ID        int64     `json:"id"`
-	Path      string    `json:"path"`
-	Clients   []string  `json:"clients"`
-	Options   string    `json:"options"`
-	CreatedAt time.Time `json:"created_at"`
-	UpdatedAt time.Time `json:"updated_at"`
+	ID           int64     `json:"id"`
+	Path         string    `json:"path"`
+	Clients      []string  `json:"clients"`
+	ReadOnly     bool      `json:"read_only"`
+	Async        bool      `json:"async"`
+	RootSquash   bool      `json:"root_squash"`
+	SubtreeCheck bool      `json:"subtree_check"`
+	FSID         int64     `json:"fsid"`
+	Advanced     string    `json:"advanced"`
+	CreatedAt    time.Time `json:"created_at"`
+	UpdatedAt    time.Time `json:"updated_at"`
+}
+
+type NFSAdvanced struct {
+	AllSquash  bool `json:"all_squash"`
+	Secure     bool `json:"secure"`
+	WDelay     bool `json:"wdelay"`
+	Hide       bool `json:"hide"`
+	Crossmnt   bool `json:"crossmnt"`
 }
 
 type DirtyModule struct {