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/backend/src/routes/editor.ts

@@ -0,0 +1,51 @@
+import { Router } from "express";
+import {
+  getDirectoryTree,
+  readFile,
+  writeFile,
+  deleteFile,
+} from "../services/editor.service.js";
+
+const router = Router();
+
+router.get("/tree/:repo", async (req, res) => {
+  try {
+    const tree = await getDirectoryTree(req.params.repo);
+    res.json(tree);
+  } catch (err: unknown) {
+    const message = err instanceof Error ? err.message : "Unknown error";
+    res.status(500).json({ error: message });
+  }
+});
+
+router.get("/file/:repo/*path", async (req, res) => {
+  try {
+    const content = await readFile(req.params.repo, req.params.path);
+    res.json({ content });
+  } catch (err: unknown) {
+    const message = err instanceof Error ? err.message : "Unknown error";
+    res.status(404).json({ error: message });
+  }
+});
+
+router.put("/file/:repo/*path", async (req, res) => {
+  try {
+    await writeFile(req.params.repo, req.params.path, req.body.content);
+    res.json({ ok: true });
+  } catch (err: unknown) {
+    const message = err instanceof Error ? err.message : "Unknown error";
+    res.status(500).json({ error: message });
+  }
+});
+
+router.delete("/file/:repo/*path", async (req, res) => {
+  try {
+    await deleteFile(req.params.repo, req.params.path);
+    res.json({ ok: true });
+  } catch (err: unknown) {
+    const message = err instanceof Error ? err.message : "Unknown error";
+    res.status(500).json({ error: message });
+  }
+});
+
+export default router;