feat(ui): establish shared desktop shell and assets

src/main/resources/static/assets/theme.js

@@ -0,0 +1,12 @@
+(() => {
+  const systemIsDark = () => window.matchMedia('(prefers-color-scheme: dark)').matches;
+  let preference = 'system';
+  try {
+    preference = localStorage.getItem('labtimesheet-theme') || 'system';
+  } catch (_) {
+    // Browser privacy settings may disable storage; the system preference remains usable.
+  }
+  const dark = preference === 'dark' || (preference === 'system' && systemIsDark());
+  document.documentElement.dataset.theme = dark ? 'dark' : 'light';
+  document.documentElement.style.colorScheme = dark ? 'dark' : 'light';
+})();