fix: correct sidebar element IDs, add missing CSS classes, and align test mocks

Three-part consistency defect identified in code review:

1. HTML: Four sidebar elements missing CSS class attributes so all CSS
   rules targeting .session-sidebar, .sidebar-toggle-btn,
   .sidebar-collapse-btn, and .sidebar-list applied to zero elements.
   Added class= attributes to all four.

2. JS: initSidebar(), toggleSidebar(), and bindSidebarClickAway() called
   $('sidebar') and $('collapse-btn') — neither ID exists in the DOM.
   Corrected to $('session-sidebar') and $('sidebar-collapse-btn').

3. Tests: initSidebar/toggleSidebar mocks matched the wrong IDs, masking
   the defect. Updated all six mock conditions to match corrected IDs.

4. Coverage gap: test_html_element_classes did not verify new sidebar
   elements carry CSS classes. Added four entries for session-sidebar,
   sidebar-toggle-btn, sidebar-collapse-btn, and sidebar-list — these
   entries would have caught Defect 1 on Task 1's test run.

All 136 JS + 167 Python tests pass. Zero regressions.

frontend/app.js

@@ -393,7 +393,7 @@ function initSidebar() {
     isOpen = window.innerWidth >= SIDEBAR_NARROW_THRESHOLD;
   }
 
-  const sidebar = $('sidebar');
+  const sidebar = $('session-sidebar');
   if (sidebar) {
     if (isOpen) {
       sidebar.classList.remove('sidebar--collapsed');
@@ -432,7 +432,7 @@ function toggleSidebar() {
   } catch (_) { /* blocked — ok */ }
 
   // Apply class
-  const sidebar = $('sidebar');
+  const sidebar = $('session-sidebar');
   if (sidebar) {
     if (isOpen) {
       sidebar.classList.remove('sidebar--collapsed');
@@ -442,7 +442,7 @@ function toggleSidebar() {
   }
 
   // Update collapse button text (‹ when open, › when closed)
-  const collapseBtn = $('collapse-btn');
+  const collapseBtn = $('sidebar-collapse-btn');
   if (collapseBtn) {
     collapseBtn.textContent = isOpen ? '\u2039' : '\u203a';
   }
@@ -461,7 +461,7 @@ function bindSidebarClickAway() {
   if (!container) return;
   container.addEventListener('click', () => {
     if (window.innerWidth >= SIDEBAR_NARROW_THRESHOLD) return;
-    const sidebar = $('sidebar');
+    const sidebar = $('session-sidebar');
     if (!sidebar) return;
     if (sidebar.classList.contains('sidebar--collapsed')) return;
     sidebar.classList.add('sidebar--collapsed');