feat(layout): add section caching with dynamic container

Refactor `layout.js` to improve section loading performance:
- Removed unused sections from the `sections` map.
- Introduced `sectionCache` to store loaded sections.
- Created a reusable `#section-container` element for managing section DOM nodes.
- Implemented lazy loading with a loading placeholder and show/hide logic for cached sections.
- Optimized CSS handling by reusing existing stylesheet links instead of removing and recreating them.
- Added safeguards to avoid reloading already‑loaded JS modules.

web/desktop/js/layout.js

@@ -3,14 +3,8 @@ const sections = {
   tasks: 'tasks/tasks.html',
   mail: 'mail/mail.html',
   chat: 'chat/chat.html',
-  dashboard: 'dashboard/dashboard.html',
-  editor: 'editor/editor.html',
-  player: 'player/player.html',
-  paper: 'paper/paper.html',
-  settings: 'settings/settings.html',
-  tables: 'tables/tables.html',
-  news: 'news/news.html'
 };
+const sectionCache = {};
 
 async function loadSectionHTML(path) {
   const response = await fetch(path);
@@ -24,24 +18,70 @@ async function switchSection(section) {
   try {
     const htmlPath = sections[section];
     console.log('Loading section:', section, 'from', htmlPath);
-    const cssPath = 
-       htmlPath.replace('.html', '.css');
+    const cssPath = htmlPath.replace('.html', '.css');
 
     // Remove any existing section CSS
     document.querySelectorAll('link[data-section-css]').forEach(link => link.remove());
 
-    // Load CSS first
-    const cssLink = document.createElement('link');
-    cssLink.rel = 'stylesheet';
-    cssLink.href = cssPath;
-    cssLink.setAttribute('data-section-css', 'true');
-    document.head.appendChild(cssLink);
+    // Load CSS first (skip if already loaded)
+    let cssLink = document.querySelector(`link[href="${cssPath}"]`);
+    if (!cssLink) {
+      cssLink = document.createElement('link');
+      cssLink.rel = 'stylesheet';
+      cssLink.href = cssPath;
+      cssLink.setAttribute('data-section-css', 'true');
+      document.head.appendChild(cssLink);
+    }
 
-    // First load HTML
-    const html = await loadSectionHTML(htmlPath);
-    mainContent.innerHTML = html;
+    // Hide previously loaded sections and show the requested one
+    // Ensure a container exists for sections
+    let container = document.getElementById('section-container');
+    if (!container) {
+      container = document.createElement('div');
+      container.id = 'section-container';
+      mainContent.appendChild(container);
+    }
 
-    // Then load JS after HTML is inserted
+    const targetDiv = document.getElementById(`section-${section}`);
+
+    if (targetDiv) {
+      // Section already loaded: hide others, show this one
+      container.querySelectorAll('.section').forEach(div => {
+        div.style.display = 'none';
+      });
+      targetDiv.style.display = 'block';
+    } else {
+      // Show loading placeholder inside the container
+      const loadingDiv = document.createElement('div');
+      loadingDiv.className = 'loading';
+      loadingDiv.textContent = 'Loading…';
+      container.appendChild(loadingDiv);
+
+      // Load HTML
+      const html = await loadSectionHTML(htmlPath);
+      // Create wrapper for the new section
+      const wrapper = document.createElement('div');
+      wrapper.id = `section-${section}`;
+      wrapper.className = 'section';
+      wrapper.innerHTML = html;
+
+      // Hide any existing sections
+      container.querySelectorAll('.section').forEach(div => {
+        div.style.display = 'none';
+      });
+
+      // Remove loading placeholder
+      container.removeChild(loadingDiv);
+
+      // Add the new section to the container and cache it
+      container.appendChild(wrapper);
+      sectionCache[section] = wrapper;
+
+      // Ensure the new section is visible
+      wrapper.style.display = 'block';
+    }
+
+    // Then load JS after HTML is inserted (skip if already loaded)
     const jsPath = htmlPath.replace('.html', '.js');
     const existingScript = document.querySelector(`script[src="${jsPath}"]`);
     if (!existingScript) {