feat(survey): lib/survey/storage — localStorage progress save/load/clear

새로고침 시 step + response 복구. SSR safe (window 체크).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/survey/storage.ts

@@ -0,0 +1,42 @@
+import type { SavedProgress } from './types';
+
+const STORAGE_KEY = 'gyeol_survey_progress_v1';
+
+/**
+ * Progress 저장. 새로고침 후 복구용.
+ * SSR 환경에서는 noop.
+ */
+export function saveProgress(progress: SavedProgress): void {
+  if (typeof window === 'undefined') return;
+  try {
+    localStorage.setItem(STORAGE_KEY, JSON.stringify(progress));
+  } catch {
+    // storage quota exceeded 등 — 무시 (응답 자체에 영향 X)
+  }
+}
+
+/**
+ * Progress 복구. 없거나 파싱 실패 시 null.
+ */
+export function loadProgress(): SavedProgress | null {
+  if (typeof window === 'undefined') return null;
+  try {
+    const raw = localStorage.getItem(STORAGE_KEY);
+    if (!raw) return null;
+    return JSON.parse(raw) as SavedProgress;
+  } catch {
+    return null;
+  }
+}
+
+/**
+ * 제출 성공 시 progress 삭제.
+ */
+export function clearProgress(): void {
+  if (typeof window === 'undefined') return;
+  try {
+    localStorage.removeItem(STORAGE_KEY);
+  } catch {
+    /* noop */
+  }
+}