feat(frontend): add progress tracking

frontend/src/lib/progress.ts

@@ -0,0 +1,40 @@
+const PROGRESS_KEY = "codetutor-progress";
+const SOLUTIONS_KEY = "codetutor-solutions";
+
+export function getCompletedQuestions(): Set<string> {
+  if (typeof window === "undefined") return new Set();
+  const stored = localStorage.getItem(PROGRESS_KEY);
+  return stored ? new Set(JSON.parse(stored)) : new Set();
+}
+
+export function markQuestionCompleted(slug: string, code?: string): void {
+  const completed = getCompletedQuestions();
+  completed.add(slug);
+  localStorage.setItem(PROGRESS_KEY, JSON.stringify([...completed]));
+
+  if (code) {
+    saveSolution(slug, code);
+  }
+}
+
+export function isQuestionCompleted(slug: string): boolean {
+  return getCompletedQuestions().has(slug);
+}
+
+export function saveSolution(slug: string, code: string): void {
+  if (typeof window === "undefined") return;
+  const solutions = getSavedSolutions();
+  solutions[slug] = code;
+  localStorage.setItem(SOLUTIONS_KEY, JSON.stringify(solutions));
+}
+
+export function getSavedSolution(slug: string): string | null {
+  if (typeof window === "undefined") return null;
+  const solutions = getSavedSolutions();
+  return solutions[slug] || null;
+}
+
+function getSavedSolutions(): Record<string, string> {
+  const stored = localStorage.getItem(SOLUTIONS_KEY);
+  return stored ? JSON.parse(stored) : {};
+}