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_nfs.go

@@ -1,11 +1,10 @@
 package web
 
 import (
+	"context"
+	"encoding/json"
 	"io"
 	"net/http"
-	"strings"
-
-	"encoding/json"
 
 	"github.com/go-chi/chi/v5"
 
@@ -17,7 +16,13 @@ import (
 type nfsExportRequest struct {
 	Path    string   `json:"path"`
 	Clients []string `json:"clients"`
-	Options string   `json:"options"`
+	// Main options
+	ReadOnly     bool `json:"read_only"`
+	Async        bool `json:"async"`
+	RootSquash   bool `json:"root_squash"`
+	SubtreeCheck bool `json:"subtree_check"`
+	// Advanced options stored as JSON string
+	Advanced string `json:"advanced"`
 }
 
 func (req nfsExportRequest) validate(allowedRoots []string) error {
@@ -29,21 +34,18 @@ func (req nfsExportRequest) validate(allowedRoots []string) error {
 			return err
 		}
 	}
-	if strings.TrimSpace(req.Options) == "" {
-		return nil
-	}
-	return system.ValidateNFSOptions(req.Options)
+	return nil
 }
 
 func (req nfsExportRequest) toModel() db.NFSExport {
-	options := strings.TrimSpace(req.Options)
-	if options == "" {
-		options = "rw,sync,no_root_squash"
-	}
 	return db.NFSExport{
-		Path:    req.Path,
-		Clients: req.Clients,
-		Options: options,
+		Path:         req.Path,
+		Clients:      req.Clients,
+		ReadOnly:     req.ReadOnly,
+		Async:        req.Async,
+		RootSquash:   req.RootSquash,
+		SubtreeCheck: req.SubtreeCheck,
+		Advanced:     req.Advanced,
 	}
 }
 
@@ -70,6 +72,22 @@ func (s *Server) handleGetNFSExport(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, http.StatusOK, export)
 }
 
+func generateFSID(ctx context.Context, dbConn *db.DB) (int64, error) {
+	used, err := dbConn.GetAllFSIDs()
+	if err != nil {
+		return 0, err
+	}
+	usedSet := make(map[uint32]struct{}, len(used))
+	for _, id := range used {
+		usedSet[uint32(id)] = struct{}{}
+	}
+	n, err := nfs.UniqueFSID(usedSet)
+	if err != nil {
+		return 0, err
+	}
+	return int64(n), nil
+}
+
 func (s *Server) handleCreateNFSExport(w http.ResponseWriter, r *http.Request) {
 	req, err := decodeNFSExportRequest(r.Body)
 	if err != nil {
@@ -81,7 +99,15 @@ func (s *Server) handleCreateNFSExport(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	export, err := s.DB.CreateNFSExport(req.toModel())
+	model := req.toModel()
+	fsid, err := generateFSID(r.Context(), s.DB)
+	if err != nil {
+		writeError(w, http.StatusInternalServerError, err.Error())
+		return
+	}
+	model.FSID = fsid
+
+	export, err := s.DB.CreateNFSExport(model)
 	if err != nil {
 		writeError(w, http.StatusInternalServerError, err.Error())
 		return
@@ -99,6 +125,12 @@ func (s *Server) handleUpdateNFSExport(w http.ResponseWriter, r *http.Request) {
 		writeError(w, http.StatusBadRequest, "invalid id")
 		return
 	}
+	existing, err := s.DB.GetNFSExport(id)
+	if err != nil {
+		writeError(w, http.StatusNotFound, err.Error())
+		return
+	}
+
 	req, err := decodeNFSExportRequest(r.Body)
 	if err != nil {
 		writeError(w, http.StatusBadRequest, err.Error())
@@ -109,7 +141,11 @@ func (s *Server) handleUpdateNFSExport(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	export, err := s.DB.UpdateNFSExport(id, req.toModel())
+	model := req.toModel()
+	model.ID = id
+	model.FSID = existing.FSID
+
+	export, err := s.DB.UpdateNFSExport(id, model)
 	if err != nil {
 		writeError(w, http.StatusNotFound, err.Error())
 		return