Menu and a second level

The project's first scene, Main.scene, becomes the menu. The menu's UI is Menu.slxml, a title and a Play button.

Create the file Assets/UI/Menu.slxml with this text, the way you created the HUD:

Assets/UI/Menu.slxml

<?xml version="1.0" encoding="UTF-8"?>
<ui version="1">
	<panel id="menu" style="position: absolute; left: 0; top: 0; width: 100%; height: 100%; flex-direction: column; align-items: center; justify-content: center">
		<panel id="title" style="margin-bottom: 32px">Coin Collector</panel>
		<!-- MainMenu finds this button by its id and loads the first level when it is clicked -->
		<button id="play" style="width: 200px" text="Play"/>
	</panel>
</ui>

Create a component named MainMenu and replace its two files:

Source/MainMenu.h

#pragma once

#include "Shalltear.h"

// the menu's Play button. Sits below the UIPanelRenderer that draws the
// menu (on the same object) -> the button exists when OnEnable looks for it
class MainMenu : public Behavior {
public:
	SLT_REFLECT_COMPONENT(MainMenu, Behavior)
	SLT_REFLECT_FIELD(firstLevel)
	SLT_END_REFLECT()

	// the scene Play loads
	Handle<SceneAsset> firstLevel;

protected:
	void OnEnable() override;
	void OnDisable() override;
};

Source/MainMenu.cpp

#include "MainMenu.h"

SLT_REGISTER_TYPE(MainMenu)

// runs when the game starts (not while you edit the scene). A component's hooks
// run only while the game plays. It wires the Play button (by the id it has in
// Menu.slxml) same way LevelController wires pause menu
void MainMenu::OnEnable() {
	UIPanelRenderer *menu = GetOwner()->GetComponent<UIPanelRenderer>().Get();
	if (!menu || !menu->GetContext()) {
		SLT_LOG_WARN("CoinCollector", "MainMenu needs a UIPanelRenderer above it on the same object");
		return;
	}
	Handle<MainMenu> self(this);
	menu->GetContext()->Events(menu->FindElement("play")).onClick = [self] {
		MainMenu *mainMenu = self.Get();
		if (SceneAsset *level = mainMenu ? mainMenu->firstLevel.Get() : nullptr) {
			SceneSystem::ReplaceScenes(level->assetId);
		}
	};
}

void MainMenu::OnDisable() {
	UIPanelRenderer *menu = GetOwner()->GetComponent<UIPanelRenderer>().Get();
	if (menu && menu->GetContext()) {
		menu->GetContext()->Events(menu->FindElement("play")).onClick = nullptr;
	}
}

Save both files. Save the level too, then double-click Main.scene in Assets/Scenes to open it.
Create an empty object named Menu. Add a UIPanelRenderer with Menu.slxml as its document and Game.slpanel as its panelSettings.
Add MainMenu below it, and drag Level1.scene onto its firstLevel field.
Save the scene.

Press Ctrl+P, then Play, and the first level starts. When it ends, you are back at the menu.

The menu in the Runtime View: the game's title and a Play button

A second level

Double-click Level1.scene to open it, and select File ▸ Save Scene As.... Change the path to project://Assets/Scenes/Level2.scene and select OK. You are now editing the copy.
Select a coin and press Ctrl+D twice, so there are seven, and move them:

Coin	Position
1	`-7, 0.75, 7`
2	`0, 0.75, 8`
3	`7, 0.75, 7`
4	`-8, 0.75, 0`
5	`8, 0.75, 0`
6	`-7, 0.75, -7`
7	`7, 0.75, -7`

The second level's seven coins in the Scene View

Create a Level Definition named Level2.level, with timeLimit 25 and coinsToWin 7, and select Save. Leave its nextLevel empty: winning the last level returns to the menu.
Select the Level object and drag Level2.level onto its level field. Save the scene.
Select Level1.level, drag Level2.scene from Assets/Scenes onto its nextLevel field and select Save.

Open Main.scene and press Ctrl+P. Play starts the first level, collecting its coins starts the second, and finishing the second returns to the menu.

Next, build the game.