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>

tests/test_server.py

@@ -103,3 +103,24 @@ async def test_reconnect_on_startup(tmp_workbench):
     assert srv._active["live-proj"]["port"] == 8070
     assert "dead-proj" not in srv._active
     assert not (tmp_workbench / "dead-proj" / ".server.json").exists()
+
+
+async def _live(server, name="p"):
+    import aiohttp
+    from workbench.project import create_project
+    create_project(name, "T", workbench_dir=server.workbench_dir)
+    port = await server._start_http_server(name)
+    return aiohttp.ClientSession(), f"http://127.0.0.1:{port}"
+
+
+@pytest.mark.asyncio
+async def test_static_blocks_path_traversal(server, tmp_workbench):
+    (tmp_workbench / "secret.txt").write_text("nope")
+    session, base = await _live(server)
+    async with session:
+        for evil in ["/../secret.txt", "/%2e%2e/secret.txt", "/..%2fsecret.txt"]:
+            async with session.get(base + evil, allow_redirects=False) as r:
+                assert r.status == 404, evil
+        async with session.get(base + "/") as r:
+            assert r.status == 200
+    await server.workbench_stop("p")