feat(ui): establish shared desktop shell and assets

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

@@ -0,0 +1,35 @@
+document.addEventListener('DOMContentLoaded', () => {
+  const root = document.documentElement;
+  const theme = document.querySelector('[data-theme-select]');
+  const stored = (() => {
+    try { return localStorage.getItem('labtimesheet-theme') || 'system'; }
+    catch (_) { return 'system'; }
+  })();
+  if (theme) {
+    theme.value = stored;
+    theme.addEventListener('change', () => {
+      try {
+        theme.value === 'system'
+          ? localStorage.removeItem('labtimesheet-theme')
+          : localStorage.setItem('labtimesheet-theme', theme.value);
+      } catch (_) {
+        // Theme still applies for this page when persistence is unavailable.
+      }
+      const dark = theme.value === 'dark'
+        || (theme.value === 'system' && matchMedia('(prefers-color-scheme: dark)').matches);
+      root.dataset.theme = dark ? 'dark' : 'light';
+      root.style.colorScheme = dark ? 'dark' : 'light';
+    });
+  }
+
+  let collapsed = false;
+  try { collapsed = localStorage.getItem('labtimesheet-sidebar') === 'collapsed'; }
+  catch (_) { /* Use the expanded default. */ }
+  root.dataset.sidebarCollapsed = String(collapsed);
+  document.querySelector('[data-sidebar-toggle]')?.addEventListener('click', () => {
+    collapsed = !collapsed;
+    root.dataset.sidebarCollapsed = String(collapsed);
+    try { localStorage.setItem('labtimesheet-sidebar', collapsed ? 'collapsed' : 'expanded'); }
+    catch (_) { /* Collapse still works for this page. */ }
+  });
+});