fix: block path traversal in static handler

The LAN-bound HTTP server joined the raw URL path onto the project dir,
so /../../.ssh/id_ed25519 (and %2e%2e variants) served any file the
user could read. Resolve and require the path stay inside the project.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/workbench/server.py

@@ -102,8 +102,10 @@ class WorkbenchServer:
         async def static_handler(request):
             proj = request.app["project_name"]
             path = request.match_info.get("path", "index.html") or "index.html"
-            file_path = project_path(proj, self.workbench_dir) / path
-            if not file_path.exists():
+            root = project_path(proj, self.workbench_dir).resolve()
+            file_path = (root / path).resolve()
+            # served on the LAN — never let "../" escape the project dir
+            if not file_path.is_relative_to(root) or not file_path.is_file():
                 return web.Response(status=404, text="Not found")
             return web.FileResponse(file_path)