Various changes.

src/Snapshot.zig

@@ -141,12 +141,12 @@ pub fn add(self: *Snapshot, tab: Tab) !void {
 /// when that isn't set — whichever GLib reports as the user's state dir.
 fn statePath(buf: []u8) ?[:0]const u8 {
     const dir = std.mem.span(glib.getUserStateDir());
-    return std.fmt.bufPrintZ(buf, "{s}/playpen/session.json", .{dir}) catch null;
+    return std.mem.printSentinel(buf, "{s}/playpen/session.json", .{dir}, 0) catch null;
 }
 
 fn stateDir(buf: []u8) ?[:0]const u8 {
     const dir = std.mem.span(glib.getUserStateDir());
-    return std.fmt.bufPrintZ(buf, "{s}/playpen", .{dir}) catch null;
+    return std.mem.printSentinel(buf, "{s}/playpen", .{dir}, 0) catch null;
 }
 
 // -------------------------------------------------------------------------