feat: add lightbox for viewing forest and reward images fullscreen

- Click any forest element or reward image to see it large
- Smooth zoom-in animation, click anywhere to close
- Hover effect on clickable images
- Works on both forest grid and reward screen

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

index.html

@@ -195,6 +195,11 @@
 			<span class="mute-btn__icon" id="mute-icon">&#x1F50A;</span>
 		</button>
 
+		<!-- Lightbox for forest element images -->
+		<div id="image-lightbox" class="lightbox" style="display:none">
+			<img id="lightbox-img" class="lightbox__img" src="" alt="">
+		</div>
+
 		<script type="module" src="/src/main.ts"></script>
 	</body>
 </html>