fix: hide MI bar on mobile to prevent overlap with rApp tab row

The MI bar (header center section) was wrapping to a full-width
third row on mobile, overlapping the rApp slider tab bar below it.
Hide both the MI bar and its header container on mobile. Users can
still access MI via Cmd+K or the floating pill.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/components/rstack-mi.ts

@@ -1100,6 +1100,7 @@ const STYLES = `
 }
 
 @media (max-width: 640px) {
+  .mi-bar { display: none; }
   .mi { max-width: none; width: 100%; }
   .mi-panel {
     position: fixed; top: 0; left: 0; right: 0; bottom: 0;

website/public/shell.css

@@ -374,8 +374,8 @@ body.rstack-sidebar-open #toolbar {
   .rstack-header__center {
     order: 3;
     flex-basis: 100%;
-    padding: 4px 0 2px;
-    display: flex;
+    padding: 0;
+    display: none;
   }
   .rstack-header__right {
     flex: 0 0 auto;