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_test.go

@@ -0,0 +1,56 @@
+package nfs
+
+import (
+	"sync"
+	"testing"
+)
+
+func TestUniqueFSIDNoCollision(t *testing.T) {
+	const goroutines = 100
+	results := make(chan uint32, goroutines)
+	var wg sync.WaitGroup
+
+	for i := 0; i < goroutines; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			n, err := generateRandomFSID()
+			if err != nil {
+				t.Errorf("generateRandomFSID failed: %v", err)
+				return
+			}
+			results <- n
+		}()
+	}
+
+	wg.Wait()
+	close(results)
+
+	seen := make(map[uint32]struct{})
+	for n := range results {
+		if _, dup := seen[n]; dup {
+			t.Errorf("duplicate fsid generated: %d", n)
+		}
+		seen[n] = struct{}{}
+	}
+}
+
+func TestUniqueFSIDWithUsedSet(t *testing.T) {
+	used := map[uint32]struct{}{
+		1: {}, 2: {}, 3: {},
+	}
+
+	for i := 0; i < 50; i++ {
+		n, err := UniqueFSID(used)
+		if err != nil {
+			t.Fatalf("UniqueFSID failed at iteration %d: %v", i, err)
+		}
+		if n == 0 {
+			t.Fatalf("fsid 0 is reserved")
+		}
+		if _, dup := used[n]; dup {
+			t.Fatalf("returned duplicate fsid: %d", n)
+		}
+		used[n] = struct{}{}
+	}
+}