feat: add file explorer with tree view and IDE layout

Backend: editor service for directory tree reading and file CRUD,
editor routes at /api/editor with path traversal protection.

Frontend: FileExplorer tree component with expand/collapse directories,
IDELayout with sidebar + header + outlet, wired into App routing.
Editor now receives state as props from App for cross-component file loading.

packages/frontend/src/layouts/IDELayout.tsx

@@ -0,0 +1,39 @@
+import { Outlet } from "react-router-dom";
+
+export function IDELayout({ sidebar }: { sidebar?: React.ReactNode }) {
+  return (
+    <div className="flex h-screen flex-col overflow-hidden" style={{ backgroundColor: "var(--forge-bg)" }}>
+      <header
+        className="flex shrink-0 items-center px-4 py-2"
+        style={{ borderBottom: "1px solid var(--forge-border)" }}
+      >
+        <h1
+          className="text-lg font-bold"
+          style={{
+            fontFamily: "'Baskerville', 'Georgia', 'Palatino', serif",
+            color: "var(--forge-accent)",
+          }}
+        >
+          Layonara Forge
+        </h1>
+      </header>
+
+      <div className="flex flex-1 overflow-hidden">
+        {sidebar && (
+          <aside
+            className="shrink-0 overflow-hidden"
+            style={{
+              width: "250px",
+              borderRight: "1px solid var(--forge-border)",
+            }}
+          >
+            {sidebar}
+          </aside>
+        )}
+        <main className="flex-1 overflow-hidden">
+          <Outlet />
+        </main>
+      </div>
+    </div>
+  );
+}