Show the walk's path on the line below its display (closes #160)

On a terminal, each draw of the walk shows the path of the last file
it found on the line below, cut short before the last column with
wide characters counted as two columns. The progressbar library's
detail row draws it. The line is erased before a warning and when the
walk ends, so the next phase is drawn on it. Nothing changes when
stderr is not a terminal.

github.com/rivo/uniseg, already in the module graph through the
progressbar library, becomes a direct dependency for the column count.

Model: opus-5-5

go.mod

@@ -3,6 +3,7 @@ module sneak.berlin/go/sfdupes
 go 1.25.7
 
 require (
+	github.com/rivo/uniseg v0.4.7
 	github.com/schollz/progressbar/v3 v3.19.1
 	github.com/spf13/cobra v1.10.2
 	golang.org/x/sys v0.46.0
@@ -18,7 +19,6 @@ require (
 	github.com/mitchellh/colorstring v0.0.0-20190213212951-d06e56a500db // indirect
 	github.com/ncruces/go-strftime v1.0.0 // indirect
 	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
-	github.com/rivo/uniseg v0.4.7 // indirect
 	github.com/spf13/pflag v1.0.9 // indirect
 	modernc.org/libc v1.74.1 // indirect
 	modernc.org/mathutil v1.7.1 // indirect