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.go

@@ -3,6 +3,7 @@ package storage
 import (
 	"context"
 	"os/exec"
+	"syscall"
 
 	"github.com/darroyo/nasctl/internal/db"
 	"github.com/darroyo/nasctl/internal/system"
@@ -30,6 +31,37 @@ func Probe(ctx context.Context) (Capabilities, error) {
 	}, nil
 }
 
+type PathStat struct {
+	Path        string  `json:"path"`
+	Available   bool    `json:"available"`
+	TotalBytes  uint64  `json:"total_bytes"`
+	UsedBytes   uint64  `json:"used_bytes"`
+	FreeBytes   uint64  `json:"free_bytes"`
+	UsedPercent float64 `json:"used_percent"`
+	Error       string  `json:"error,omitempty"`
+}
+
+func StatPath(p string) PathStat {
+	out := PathStat{Path: p, Available: false}
+	if p == "" {
+		out.Error = "path no configurado"
+		return out
+	}
+	var st syscall.Statfs_t
+	if err := syscall.Statfs(p, &st); err != nil {
+		out.Error = err.Error()
+		return out
+	}
+	out.Available = true
+	out.TotalBytes = st.Blocks * uint64(st.Bsize)
+	out.FreeBytes = st.Bavail * uint64(st.Bsize)
+	out.UsedBytes = out.TotalBytes - out.FreeBytes
+	if out.TotalBytes > 0 {
+		out.UsedPercent = float64(out.UsedBytes) / float64(out.TotalBytes) * 100
+	}
+	return out
+}
+
 func ValidatePaths(cfg db.StorageConfig) error {
 	return nil
 }