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/index.ts

@@ -6,6 +6,7 @@ import { fileURLToPath } from "url";
 import { initWebSocket, getClientCount } from "./services/ws.service.js";
 import workspaceRouter from "./routes/workspace.js";
 import dockerRouter from "./routes/docker.js";
+import editorRouter from "./routes/editor.js";
 
 const __dirname = path.dirname(fileURLToPath(import.meta.url));
 const app = express();
@@ -26,6 +27,7 @@ app.get("/api/health", (_req, res) => {
 
 app.use("/api/workspace", workspaceRouter);
 app.use("/api/docker", dockerRouter);
+app.use("/api/editor", editorRouter);
 
 const frontendDist = path.resolve(__dirname, "../../frontend/dist");
 app.use(express.static(frontendDist));