fix: resolve initialization bugs breaking level indicator and expected preview

- Fix renderLesson to handle null inputEl (CodeMirror replaces textarea)
- Fix renderExpectedPreview to use 'solution' property instead of 'solutionCode'
- These fixes restore level indicator, expected preview, and auto-run functionality

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/helpers/renderer.js

@@ -133,10 +133,10 @@ export function renderLesson(titleEl, descriptionEl, taskEl, previewEl, prefixEl
 	// Set task instructions
 	taskEl.innerHTML = lesson.task || "";
 
-	// Set code editor contents
-	// prefixEl.textContent = lesson.codePrefix || "";
-	inputEl.value = lesson.initialCode || "";
-	// suffixEl.textContent = lesson.codeSuffix || "";
+	// Set code editor contents (if inputEl is provided)
+	if (inputEl) {
+		inputEl.value = lesson.initialCode || "";
+	}
 
 	// Clear any existing feedback
 	clearFeedback();