fix(dashboard): suppress unicode-animations postinstall during npm ci

Set CI=1 in _run_npm_install_deterministic so the package's /dev/tty
postinstall demo is skipped during hermes dashboard web UI builds.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/hermes_cli/test_web_ui_build.py

@@ -140,6 +140,22 @@ class TestBuildWebUISkipsWhenFresh:
         assert kwargs["encoding"] == "utf-8"
         assert kwargs["errors"] == "replace"
 
+    def test_npm_install_sets_ci_to_suppress_postinstall_tty_output(self, tmp_path):
+        web_dir, _ = _make_web_dir(tmp_path)
+        (web_dir / "package-lock.json").write_text("{}", encoding="utf-8")
+
+        mock_cp = __import__("subprocess").CompletedProcess([], 0, stdout="", stderr="")
+        with patch("hermes_cli.main.subprocess.run", return_value=mock_cp) as mock_run:
+            _run_npm_install_deterministic(
+                "/usr/bin/npm",
+                web_dir,
+                env={"PYTHON": "/nix/store/python"},
+            )
+
+        _, kwargs = mock_run.call_args
+        assert kwargs["env"]["CI"] == "1"
+        assert kwargs["env"]["PYTHON"] == "/nix/store/python"
+
     def test_npm_install_uses_workspace_web_scope(self, tmp_path):
         web_dir, _ = _make_web_dir(tmp_path)
         # Real workspace checkout: the single lockfile lives at the root, so