fix(split-pane): hide the Split header button when showSplitButton is off

.btn-split--hidden had no matching CSS rule anywhere, so the opt-in Split
header button shipped visible to every user on every viewport regardless of
the setting. Add the `display: none !important` rule alongside its sibling
marker classes (.btn-multimonitor--hidden etc.), plus a static regression
guard (test/split-pane-hidden-button-css.test.ts) that fails if any future
"*--hidden" marker class in index.html is missing a matching CSS rule.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/web/public/styles.css

@@ -2282,6 +2282,13 @@ html[data-tab-orientation='vertical'] .tab-rail .session-tab .tab-name-prefix {
   display: none !important;
 }
 
+/* Split-pane header button: hidden by default (opt-in via App Settings →
+   Header & Panels → Header buttons → Split, `showSplitButton`). Pure
+   client-side toggle applied by applyHeaderVisibilitySettings(). */
+.btn-split--hidden {
+  display: none !important;
+}
+
 .btn-icon-header.btn-settings {
   width: 30px;
   height: 30px;