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/web/handlers_system.go

@@ -1,7 +1,10 @@
 package web
 
 import (
+	"bufio"
 	"net/http"
+	"os"
+	"sort"
 	"strings"
 	"syscall"
 
@@ -9,11 +12,15 @@ import (
 )
 
 type diskUsage struct {
-	Path        string  `json:"path"`
-	TotalBytes  uint64  `json:"total_bytes"`
-	FreeBytes   uint64  `json:"free_bytes"`
-	UsedBytes   uint64  `json:"used_bytes"`
-	UsedPercent float64 `json:"used_percent"`
+	Path        string   `json:"path"`
+	TotalBytes  uint64   `json:"total_bytes"`
+	FreeBytes   uint64   `json:"free_bytes"`
+	UsedBytes   uint64   `json:"used_bytes"`
+	UsedPercent float64  `json:"used_percent"`
+	Source      string   `json:"source"`
+	UsedBy      []string `json:"used_by,omitempty"`
+	Available   bool     `json:"available"`
+	Error       string   `json:"error,omitempty"`
 }
 
 type serviceStatus struct {
@@ -24,39 +31,97 @@ type serviceStatus struct {
 
 func (s *Server) handleSystemStatus(w http.ResponseWriter, r *http.Request) {
 	status := map[string]any{
-		"disks":    collectDiskUsage(),
-		"services": collectServiceStatus(r),
+		"disks":    s.collectDiskUsage(),
+		"services": s.collectServiceStatus(r),
 	}
 	writeJSON(w, http.StatusOK, status)
 }
 
-func collectDiskUsage() []diskUsage {
-	paths := []string{"/"}
-	var usages []diskUsage
-	for _, path := range paths {
-		var stat syscall.Statfs_t
-		if err := syscall.Statfs(path, &stat); err != nil {
-			continue
+func (s *Server) collectDiskUsage() []diskUsage {
+	entries := make(map[string]*diskUsage)
+
+	entries["/"] = &diskUsage{Path: "/", Source: "system", Available: true}
+
+	shares, err := s.DB.ListSambaShares()
+	if err == nil {
+		for _, share := range shares {
+			key := share.Path
+			if e, ok := entries[key]; ok {
+				e.UsedBy = append(e.UsedBy, share.Name)
+			} else {
+				entries[key] = &diskUsage{Path: key, Source: "samba", UsedBy: []string{share.Name}, Available: true}
+			}
 		}
-		total := stat.Blocks * uint64(stat.Bsize)
-		free := stat.Bavail * uint64(stat.Bsize)
-		used := total - free
-		var pct float64
-		if total > 0 {
-			pct = float64(used) / float64(total) * 100
-		}
-		usages = append(usages, diskUsage{
-			Path:        path,
-			TotalBytes:  total,
-			FreeBytes:   free,
-			UsedBytes:   used,
-			UsedPercent: pct,
-		})
 	}
-	return usages
+
+	exports, err := s.DB.ListNFSExports()
+	if err == nil {
+		for _, exp := range exports {
+			key := exp.Path
+			label := "nfs:" + exp.Path
+			if e, ok := entries[key]; ok {
+				e.UsedBy = append(e.UsedBy, label)
+			} else {
+				entries[key] = &diskUsage{Path: key, Source: "nfs", UsedBy: []string{label}, Available: true}
+			}
+		}
+	}
+
+	readProcMounts(entries)
+
+	var result []diskUsage
+	for _, e := range entries {
+		fillDiskUsage(e)
+		result = append(result, *e)
+	}
+
+	sort.Slice(result, func(i, j int) bool {
+		if result[i].Source != result[j].Source {
+			return result[i].Source < result[j].Source
+		}
+		return result[i].Path < result[j].Path
+	})
+
+	return result
 }
 
-func collectServiceStatus(r *http.Request) []serviceStatus {
+func readProcMounts(entries map[string]*diskUsage) {
+	f, err := os.Open("/proc/mounts")
+	if err != nil {
+		return
+	}
+	defer f.Close()
+
+	scanner := bufio.NewScanner(f)
+	for scanner.Scan() {
+		fields := strings.Fields(scanner.Text())
+		if len(fields) < 3 {
+			continue
+		}
+		mountPoint := fields[1]
+		if _, ok := entries[mountPoint]; !ok {
+			entries[mountPoint] = &diskUsage{Path: mountPoint, Source: "mount", Available: true}
+		}
+	}
+}
+
+func fillDiskUsage(e *diskUsage) {
+	var stat syscall.Statfs_t
+	if err := syscall.Statfs(e.Path, &stat); err != nil {
+		e.Available = false
+		e.Error = "path no disponible"
+		return
+	}
+	e.Available = true
+	e.TotalBytes = stat.Blocks * uint64(stat.Bsize)
+	e.FreeBytes = stat.Bavail * uint64(stat.Bsize)
+	e.UsedBytes = e.TotalBytes - e.FreeBytes
+	if e.TotalBytes > 0 {
+		e.UsedPercent = float64(e.UsedBytes) / float64(e.TotalBytes) * 100
+	}
+}
+
+func (s *Server) collectServiceStatus(r *http.Request) []serviceStatus {
 	services := []string{"smbd", "nfs-server"}
 	var statuses []serviceStatus
 	for _, name := range services {