feat: persist circuit and components to localStorage

Auto-saves every 3 seconds and on page unload. Restores the full
state (circuit, camera, custom components) on page load.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

js/saveLoad.js

@@ -2,6 +2,8 @@
 import { state } from './state.js';
 import { progress } from './levels.js';
 
+const STORAGE_KEY = 'logiclab_state';
+
 /**
  * Save complete application state to JSON
  */
@@ -147,3 +149,46 @@ export async function pasteFromClipboard() {
         return { success: false, error: e.message };
     }
 }
+
+/**
+ * Save state to localStorage
+ */
+export function saveToStorage() {
+    try {
+        const data = saveState();
+        localStorage.setItem(STORAGE_KEY, JSON.stringify(data));
+    } catch (e) {
+        console.warn('[storage] failed to save:', e.message);
+    }
+}
+
+/**
+ * Load state from localStorage (returns true if state was restored)
+ */
+export function loadFromStorage() {
+    try {
+        const json = localStorage.getItem(STORAGE_KEY);
+        if (!json) return false;
+        const data = JSON.parse(json);
+        const result = loadState(data);
+        if (result.success) {
+            console.log('[storage] restored state from localStorage');
+        }
+        return result.success;
+    } catch (e) {
+        console.warn('[storage] failed to load:', e.message);
+        return false;
+    }
+}
+
+/**
+ * Start auto-saving to localStorage on an interval
+ */
+let autoSaveInterval = null;
+export function startAutoSave(intervalMs = 3000) {
+    if (autoSaveInterval) clearInterval(autoSaveInterval);
+    autoSaveInterval = setInterval(saveToStorage, intervalMs);
+    // Also save on page unload
+    window.addEventListener('beforeunload', saveToStorage);
+    console.log(`[storage] auto-save enabled (every ${intervalMs}ms)`);
+}