Various changes.

src/Settings.zig

@@ -740,12 +740,12 @@ fn serialize(self: *Settings) SaveError![]u8 {
 /// `~/.config/playpen/settings.json` when that isn't set.
 fn configPath(buf: []u8) ?[:0]const u8 {
     const dir = std.mem.span(glib.getUserConfigDir());
-    return std.fmt.bufPrintZ(buf, "{s}/playpen/settings.json", .{dir}) catch null;
+    return std.mem.printSentinel(buf, "{s}/playpen/settings.json", .{dir}, 0) catch null;
 }
 
 fn configDir(buf: []u8) ?[:0]const u8 {
     const dir = std.mem.span(glib.getUserConfigDir());
-    return std.fmt.bufPrintZ(buf, "{s}/playpen", .{dir}) catch null;
+    return std.mem.printSentinel(buf, "{s}/playpen", .{dir}, 0) catch null;
 }
 
 // -------------------------------------------------------------------------