fix(element/scrollable): support deriving Container size of scrollable

examples/demo.zig

@@ -59,7 +59,9 @@ pub fn main() !void {
         .layout = .{
             .gap = 1,
             .padding = .vertical(2),
+            .direction = .vertical,
         },
+        .size = .{ .y = 90 },
     }, .{});
     try box.append(try App.Container.init(allocator, .{
         .rectangle = .{ .fill = .light_green },
@@ -72,10 +74,7 @@ pub fn main() !void {
     }, .{}));
     defer box.deinit();
 
-    var scrollable: App.Scrollable = .{
-        .container = box,
-        .min_size = .{ .x = 60 },
-    };
+    var scrollable: App.Scrollable = .{ .container = box };
 
     var container = try App.Container.init(allocator, .{
         .layout = .{
@@ -92,7 +91,7 @@ pub fn main() !void {
             .color = .light_blue,
             .sides = .all,
         },
-        .size = .{ .x = 200 },
+        .size = .{ .x = 100 },
     }, .{}));
     try container.append(try App.Container.init(allocator, .{
         .rectangle = .{ .fill = .blue },