fix: use exec.LookPath instead of 'command -v' for binary detection

'command' is a shell builtin, not a real binary, so exec.Command
always failed with exit 127. Replaced with os/exec.LookPath which
correctly resolves binaries in PATH.

Also adds capabilities_test.go to verify Probe() matches
exec.LookPath results.

Version: 0.7.2

internal/storage/capabilities.go

@@ -2,6 +2,7 @@ package storage
 
 import (
 	"context"
+	"os/exec"
 
 	"github.com/darroyo/nasctl/internal/db"
 	"github.com/darroyo/nasctl/internal/system"
@@ -15,17 +16,17 @@ type Capabilities struct {
 }
 
 func Probe(ctx context.Context) (Capabilities, error) {
-	rsyncOut, _, _ := system.Run(ctx, "command", "-v", "rsync")
-	snapraidOut, _, _ := system.Run(ctx, "command", "-v", "snapraid")
-	mergerfsOut, _, _ := system.Run(ctx, "command", "-v", "mergerfs")
+	_, rsyncErr := exec.LookPath("rsync")
+	_, snapraidErr := exec.LookPath("snapraid")
+	_, mergerfsErr := exec.LookPath("mergerfs")
 
 	_, _, mergerfsMountedErr := system.Run(ctx, "grep", "-q", "fuse.mergerfs", "/proc/mounts")
 
 	return Capabilities{
-		Rsync:           len(rsyncOut) > 0,
-		MergerfsBin:     len(mergerfsOut) > 0,
+		Rsync:           rsyncErr == nil,
+		MergerfsBin:     mergerfsErr == nil,
 		MergerfsMounted: mergerfsMountedErr == nil,
-		SnapraidBin:     len(snapraidOut) > 0,
+		SnapraidBin:     snapraidErr == nil,
 	}, nil
 }