fix(shell): responsive subnav and tabbar on mobile

- Tab row: set top: 0 on mobile (was inheriting top: 56px from desktop
  fixed positioning while being sticky on mobile)
- Subnav + tabbar: switch from sticky to relative on mobile so they
  scroll naturally instead of fighting with the sticky header
- Smaller pill padding/font on ≤480px screens

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

server/shell.ts

@@ -1333,7 +1333,18 @@ const SUBNAV_CSS = `
 }
 .rapp-subnav::-webkit-scrollbar { display: none; }
 @media (max-width: 640px) {
-  .rapp-subnav { top: 0; }
+  .rapp-subnav {
+    position: relative;
+    top: auto;
+    padding: 0.375rem 0.75rem;
+    gap: 0.25rem;
+  }
+}
+@media (max-width: 480px) {
+  .rapp-nav-pill {
+    padding: 0.25rem 0.5rem;
+    font-size: 0.75rem;
+  }
 }
 .rapp-nav-pill {
   padding: 0.3125rem 0.75rem;
@@ -1410,7 +1421,12 @@ const TABBAR_CSS = `
 }
 .rapp-tabbar::-webkit-scrollbar { display: none; }
 @media (max-width: 640px) {
-  .rapp-tabbar { top: 0; }
+  .rapp-tabbar {
+    position: relative;
+    top: auto;
+    padding: 0.25rem 0.75rem;
+    gap: 0.25rem;
+  }
 }
 `;
 

website/public/shell.css

@@ -392,6 +392,7 @@ body.rstack-sidebar-open #toolbar {
   }
   .rstack-tab-row {
     position: sticky;
+    top: 0;
   }
   #app {
     padding-top: 0;