Keep a non-root user's database in the XDG state directory (closes #171)

Without SFDUPES_DATABASE, a user other than root now uses
sfdupes/db.sqlite under $XDG_STATE_HOME (~/.local/state by default),
which scan creates, instead of failing on /var/lib/sfdupes. Root keeps
/var/lib/sfdupes/db.sqlite. sfdupes --help and README say where the
database is. Adds github.com/adrg/xdg, the decided XDG library.

Model: opus-5-5

go.mod

@@ -3,6 +3,7 @@ module sneak.berlin/go/sfdupes
 go 1.25.7
 
 require (
+	github.com/adrg/xdg v0.5.3
 	github.com/schollz/progressbar/v3 v3.19.1
 	github.com/spf13/cobra v1.10.2
 	golang.org/x/sys v0.46.0