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.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

src/app.js

@@ -149,7 +149,6 @@ const elements = {
 	expectedOverlay: document.getElementById("expected-overlay"),
 	previewWrapper: document.querySelector(".preview-wrapper"),
 	previewSection: document.querySelector(".preview-section"),
-	backBtn: document.getElementById("back-btn"),
 	prevBtn: document.getElementById("prev-btn"),
 	nextBtn: document.getElementById("next-btn"),
 	gameControls: document.querySelector(".game-controls"),
@@ -751,13 +750,18 @@ function updateNavigationButtons() {
 	const engineState = lessonEngine.getCurrentState();
 	const isPlayground = engineState.lesson?.mode === "playground";
 
-	// Hide nav buttons and center controls in playground mode, show back button
-	elements.backBtn?.classList.toggle("hidden", !isPlayground);
-	elements.prevBtn.classList.toggle("hidden", isPlayground);
+	// In playground mode: hide next button, repurpose prev as back button
 	elements.nextBtn.classList.toggle("hidden", isPlayground);
 	elements.gameControls?.classList.toggle("centered", isPlayground);
 
-	if (!isPlayground) {
+	if (isPlayground) {
+		// Change prev button to "Back" in playground mode
+		elements.prevBtn.textContent = t("back");
+		elements.prevBtn.disabled = false;
+		elements.prevBtn.classList.remove("btn-disabled");
+	} else {
+		// Normal mode: prev/next navigation
+		elements.prevBtn.textContent = t("previous");
 		elements.prevBtn.disabled = !engineState.canGoPrev;
 		elements.nextBtn.disabled = !engineState.canGoNext;
 
@@ -788,7 +792,16 @@ function nextLesson() {
 }
 
 function prevLesson() {
-	const prevModuleId = lessonEngine.getCurrentState().module?.id;
+	const engineState = lessonEngine.getCurrentState();
+	const isPlayground = engineState.lesson?.mode === "playground";
+
+	// In playground mode, "Back" navigates to home
+	if (isPlayground) {
+		navigateTo("#");
+		return;
+	}
+
+	const prevModuleId = engineState.module?.id;
 	const success = lessonEngine.previousLesson();
 	if (success) {
 		const newState = lessonEngine.getCurrentState();
@@ -2455,12 +2468,12 @@ function init() {
 	// Set timeout to show fallback if loading takes too long
 	loadingTimeout = setTimeout(showLoadingFallback, 3000);
 
-	// Load modules after editor is ready
-	initializeModules();
-
-	// Handle OAuth callback BEFORE router (tokens are in URL hash)
+	// Handle OAuth callback FIRST (tokens are in URL hash, must run before router)
 	handleOAuthCallback().then(() => {
-		// Initialize URL router for shareable links
+		// Load modules (this also calls handleRoute inside)
+		initializeModules();
+
+		// Initialize URL router for browser back/forward
 		initRouter();
 
 		// Initialize authentication

src/auth.js

@@ -17,6 +17,7 @@ export async function handleOAuthCallback() {
 
   // Check if hash contains OAuth tokens (access_token, error, etc.)
   if (!hash.includes("access_token") && !hash.includes("error_description") && !hash.includes("refresh_token")) {
+    console.log("[Auth] No OAuth tokens in hash");
     return false;
   }
 
@@ -25,20 +26,33 @@ export async function handleOAuthCallback() {
   try {
     const supabaseModule = await import("./supabase.js");
     if (!supabaseModule.isConfigured) {
+      console.log("[Auth] Supabase not configured");
       return false;
     }
 
-    // Let Supabase process the OAuth tokens
-    const { data, error } = await supabaseModule.auth.getSession();
+    // Parse tokens from hash
+    const params = new URLSearchParams(hash.substring(1));
+    const accessToken = params.get("access_token");
+    const refreshToken = params.get("refresh_token");
 
-    if (error) {
-      console.error("[Auth] OAuth callback error:", error.message);
-    } else if (data?.session) {
-      console.log("[Auth] OAuth login successful:", data.session.user?.email);
-      oauthHandled = true;
+    console.log("[Auth] Tokens found:", { accessToken: !!accessToken, refreshToken: !!refreshToken });
+
+    if (accessToken && refreshToken) {
+      // Explicitly set the session with tokens from URL
+      const { data, error } = await supabaseModule.auth.setSession({
+        access_token: accessToken,
+        refresh_token: refreshToken
+      });
+
+      if (error) {
+        console.error("[Auth] OAuth setSession error:", error.message);
+      } else if (data?.session) {
+        console.log("[Auth] OAuth login successful:", data.session.user?.email);
+        oauthHandled = true;
+      }
     }
 
-    // Clear the hash after processing (will be replaced by router)
+    // Clear the hash after processing
     window.history.replaceState(null, "", window.location.pathname);
 
     return true;

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>

src/supabase.js

@@ -45,6 +45,10 @@ export const auth = {
     supabase?.auth.getSession() ??
     Promise.resolve({ data: { session: null }, error: null }),
 
+  setSession: ({ access_token, refresh_token }) =>
+    supabase?.auth.setSession({ access_token, refresh_token }) ??
+    Promise.resolve({ data: { session: null }, error: { message: "Not configured" } }),
+
   onAuthStateChange: (callback) =>
     supabase?.auth.onAuthStateChange(callback) ?? { data: { subscription: { unsubscribe: () => {} } } },