fix: use same button position/style for playground back button

Instead of a separate back button, the Previous button is repurposed
as "Back" in playground mode - same position (left), same style.
Only the Next button is hidden in playground mode.

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src/index.html

@@ -434,7 +434,6 @@
 				<!-- Right Panel: Preview + Navigation -->
 				<div class="right-panel">
 					<div class="game-controls">
-						<a id="back-btn" href="#" class="btn hidden" data-i18n="back">Back</a>
 						<button id="prev-btn" class="btn" data-i18n="previous">Previous</button>
 						<span class="module-pill" id="module-pill">
 							<span class="module-name"></span>