fix: make web UI build output decoding robust on Windows

On Windows systems using a Chinese GBK locale, `hermes update` could misreport the Web UI build as failed even when `npm run build` actually succeeded. The failure was caused by Python decoding captured npm output with the process locale inside a background subprocess reader thread. When npm emitted bytes such as `0x85`, decoding under GBK raised `UnicodeDecodeError`, and Hermes then surfaced a misleading "Web UI build failed" warning.

This change makes the npm install/npm ci path and the Web UI build step decode captured output explicitly as UTF-8 with `errors="replace"`. That keeps unexpected bytes from crashing output collection, preserves successful builds, and prevents false negatives during update on Windows.

The patch also adds regression tests that verify these subprocess calls always use explicit UTF-8 decoding with replacement semantics.

tests/hermes_cli/test_web_ui_build.py

@@ -13,7 +13,7 @@ from unittest.mock import patch
 
 import pytest
 
-from hermes_cli.main import _web_ui_build_needed, _build_web_ui
+from hermes_cli.main import _web_ui_build_needed, _build_web_ui, _run_npm_install_deterministic
 
 
 def _touch(path: Path, offset: float = 0.0) -> None:
@@ -119,3 +119,31 @@ class TestBuildWebUISkipsWhenFresh:
 
         assert result is True
         assert mock_run.call_count == 2  # npm install + npm run build
+
+    def test_npm_install_uses_utf8_replace_output_decoding(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:
+            result = _run_npm_install_deterministic("/usr/bin/npm", web_dir)
+
+        assert result.returncode == 0
+        _, kwargs = mock_run.call_args
+        assert kwargs["text"] is True
+        assert kwargs["encoding"] == "utf-8"
+        assert kwargs["errors"] == "replace"
+
+    def test_web_build_uses_utf8_replace_output_decoding(self, tmp_path):
+        web_dir, _ = _make_web_dir(tmp_path)
+
+        mock_cp = __import__("subprocess").CompletedProcess([], 0, stdout="", stderr="")
+        with patch("hermes_cli.main.shutil.which", return_value="/usr/bin/npm"), \
+             patch("hermes_cli.main.subprocess.run", side_effect=[mock_cp, mock_cp]) as mock_run:
+            result = _build_web_ui(web_dir)
+
+        assert result is True
+        _, build_kwargs = mock_run.call_args_list[1]
+        assert build_kwargs["text"] is True
+        assert build_kwargs["encoding"] == "utf-8"
+        assert build_kwargs["errors"] == "replace"