fix(tabs): hide a case box the sidebar filter emptied (#580)

Since the sidebar filter and the rail search share _applyTabListFilter(),
the sidebar's filter box also marks a .tab-cluster it emptied in the
by-case tab layout and rewrites its count. But the only rule hiding
.tab-cluster.tab-filtered-out was the rail-scoped one, so in the sidebar
the emptied box stayed painted with its label and a count of 0 while its
rows were hidden.

The sidebar rule now hides the marked case box as well. It stays scoped
to html[data-session-list="sidebar"], so a leaked class still cannot hide
anything on the header strip. An alerted row still counts toward its box
(the owner's call on #580), so a box holding a row that needs the user
never hides.

Tests: a unit test renders the sidebar in the by-case layout and checks
the emptied box is marked with a 0 count, that an alerted row keeps its
box on screen and counted, and that clearing restores the totals; a
stylesheet check pins the scoped selector; the browser test measures the
shipped styles.css in Chromium: the marked box is not painted in the
sidebar layout, an unmarked one is, and the header layout hides nothing.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/web/public/styles.css

@@ -18814,8 +18814,12 @@ html[data-tab-orientation='vertical'] .tab-rail .session-tab.drag-over-right {
    Scoped to the sidebar layout on purpose: applySidebarFilter() already strips
    the class whenever the filter box is off screen, and this prefix is the
    second lock — a leaked class must never be able to hide tabs from the header
-   strip, which has no filter control to clear it with. */
-html[data-session-list="sidebar"] .session-tab.tab-filtered-out {
+   strip, which has no filter control to clear it with. The shared filter
+   (_applyTabListFilter) also marks a case box (tabArrangement 'case') the
+   filter emptied, which hides with its label and count rather than staying
+   on screen reading 0. */
+html[data-session-list="sidebar"] .session-tab.tab-filtered-out,
+html[data-session-list="sidebar"] .tab-cluster.tab-filtered-out {
   display: none !important;
 }