feat: prevent easy reloading

ts/src/browser.ts

@@ -793,4 +793,8 @@ m.mount(document.getElementById("sec-meta")!,      MetaPanel);
 // Expose globals referenced by topbar inline handlers
 Object.assign(window, { loadBin, exportBin, addSection, cycleTheme });
 
+window.addEventListener("beforeunload", (e) => {
+  if (isDirty) e.preventDefault();
+});
+
 triggerPreview();