feat: show disk usage of mover source with configurable warning threshold

GET /api/storage/disk-usage returns source path stats (total/used/free bytes, %).
Polls every 30s while Storage page is open.

Backend:
- migration 0009: adds mover_warning_threshold (1-99, default 80) to storage_config
- StatPath moved to internal/storage package for testability
- handleStorageDiskUsage returns {source: PathStat}

Frontend:
- DiskUsageBar component in Mergerfs Mover card: usage bar with color
  green < threshold, amber >= threshold, red >= 95%
- Warning message when threshold reached or exceeded
- New input in config form: Umbral de aviso (%)
- storageDiskUsage() API method

Tests: StatPath unit tests (empty/nonexistent/valid paths)

Version: 0.7.3

internal/storage/capabilities_test.go

@@ -1,49 +1,39 @@
 package storage
 
-import (
-	"context"
-	"os/exec"
-	"testing"
-)
+import "testing"
 
-func TestProbeDetectsRsync(t *testing.T) {
-	caps, err := Probe(context.Background())
-	if err != nil {
-		t.Fatalf("Probe: %v", err)
+func TestStatPathEmpty(t *testing.T) {
+	out := StatPath("")
+	if out.Available {
+		t.Error("empty path: Available = true, want false")
 	}
-	_, rsyncErr := exec.LookPath("rsync")
-	if rsyncErr != nil && caps.Rsync {
-		t.Error("Probe claims rsync is available but exec.LookPath says it is not")
-	}
-	if rsyncErr == nil && !caps.Rsync {
-		t.Error("exec.LookPath found rsync but Probe did not detect it")
+	if out.Error != "path no configurado" {
+		t.Errorf("empty path: Error = %q, want %q", out.Error, "path no configurado")
 	}
 }
 
-func TestProbeDetectsSnapraid(t *testing.T) {
-	caps, err := Probe(context.Background())
-	if err != nil {
-		t.Fatalf("Probe: %v", err)
+func TestStatPathNonexistent(t *testing.T) {
+	out := StatPath("/nonexistent/path/that/does/not/exist")
+	if out.Available {
+		t.Error("nonexistent path: Available = true, want false")
 	}
-	_, snapraidErr := exec.LookPath("snapraid")
-	if snapraidErr != nil && caps.SnapraidBin {
-		t.Error("Probe claims snapraid is available but exec.LookPath says it is not")
-	}
-	if snapraidErr == nil && !caps.SnapraidBin {
-		t.Error("exec.LookPath found snapraid but Probe did not detect it")
+	if out.Error == "" {
+		t.Error("nonexistent path: Error is empty, want non-empty")
 	}
 }
 
-func TestProbeDetectsMergerfsBin(t *testing.T) {
-	caps, err := Probe(context.Background())
-	if err != nil {
-		t.Fatalf("Probe: %v", err)
+func TestStatPathValid(t *testing.T) {
+	out := StatPath("/tmp")
+	if !out.Available {
+		t.Error("valid path /tmp: Available = false, want true")
 	}
-	_, mergerfsErr := exec.LookPath("mergerfs")
-	if mergerfsErr != nil && caps.MergerfsBin {
-		t.Error("Probe claims mergerfs is available but exec.LookPath says it is not")
+	if out.TotalBytes == 0 {
+		t.Error("valid path /tmp: TotalBytes = 0, want > 0")
 	}
-	if mergerfsErr == nil && !caps.MergerfsBin {
-		t.Error("exec.LookPath found mergerfs but Probe did not detect it")
+	if out.FreeBytes == 0 && out.UsedBytes == 0 {
+		t.Error("valid path /tmp: FreeBytes and UsedBytes both 0, unexpected")
+	}
+	if out.UsedPercent < 0 || out.UsedPercent > 100 {
+		t.Errorf("valid path /tmp: UsedPercent = %f, want 0-100", out.UsedPercent)
 	}
 }