feat: add minimal frontend (accounts, trial balance)

frontend/js/api.js

@@ -0,0 +1,10 @@
+const API_BASE = "http://localhost:18080";
+
+async function apiGet(path) {
+  const res = await fetch(`${API_BASE}${path}`);
+  if (!res.ok) {
+    const text = await res.text();
+    throw new Error(text || "APIエラー");
+  }
+  return res.json();
+}