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.

coordinator/tests/test_frontend_html.py

@@ -247,6 +247,14 @@ def test_html_element_classes() -> None:
         ),
         ("session-pill-label", "session-pill__label", "needs max-width truncation"),
         ("session-pill-bell", "session-pill__bell", "needs amber var(--bell) color"),
+        (
+            "session-sidebar",
+            "session-sidebar",
+            "flex-column layout and collapse transition",
+        ),
+        ("sidebar-toggle-btn", "sidebar-toggle-btn", "36x36 bordered button styles"),
+        ("sidebar-collapse-btn", "sidebar-collapse-btn", "chevron button hover styles"),
+        ("sidebar-list", "sidebar-list", "flex:1 overflow-y:auto scroll container"),
     ]
     for el_id, expected_class, reason in cases:
         el = soup.find(id=el_id)