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/services/api.ts

@@ -12,9 +12,29 @@ async function request<T>(path: string, options?: RequestInit): Promise<T> {
   return res.json();
 }
 
+export interface FileNode {
+  name: string;
+  path: string;
+  type: "file" | "directory";
+  children?: FileNode[];
+}
+
 export const api = {
   health: () => request<{ status: string; wsClients: number }>("/health"),
 
+  editor: {
+    tree: (repo: string) => request<FileNode[]>(`/editor/tree/${repo}`),
+    readFile: (repo: string, filePath: string) =>
+      request<{ content: string }>(`/editor/file/${repo}/${filePath}`),
+    writeFile: (repo: string, filePath: string, content: string) =>
+      request(`/editor/file/${repo}/${filePath}`, {
+        method: "PUT",
+        body: JSON.stringify({ content }),
+      }),
+    deleteFile: (repo: string, filePath: string) =>
+      request(`/editor/file/${repo}/${filePath}`, { method: "DELETE" }),
+  },
+
   workspace: {
     getConfig: () => request<Record<string, unknown>>("/workspace/config"),
     updateConfig: (data: Record<string, unknown>) =>