add(sizing): grow configuration

Currently the 'grid' and 'mixed' examples are not working yet.

examples/styles/palette.zig

@@ -64,7 +64,7 @@ pub fn main() !void {
         if (comptime field.value == 0) continue; // zterm.Color.default == 0 -> skip
         try box.append(try App.Container.init(allocator, .{ .rectangle = .{ .fill = @enumFromInt(field.value) } }, .{}));
     }
-    var scrollable: App.Scrollable = .init(box, .{ .x = 3 * std.meta.fields(zterm.Color).len }); // ensure enough columns to render all colors -> scrollable otherwise
+    var scrollable: App.Scrollable = .{ .container = box };
     try container.append(try App.Container.init(allocator, .{}, scrollable.element()));
 
     try app.start();