fix(mobile): hide Split button + single-pane navigator chrome

v1.6 left the Workspace's Split-button row visible on mobile even
when only one pane was open — ~36px of dead chrome above the chat.
Wrap the entire Split-row in !isMobile so mobile gets header → chat
with no intermediate strip. The existing mobile pane-navigator strip
(gated to panes.length > 1) is unchanged and still appears once a
second pane is created via the long-press "New chat" menu item (G3).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/web/src/components/Workspace.tsx

@@ -125,6 +125,7 @@ export function Workspace({ sessionId, projectId }: Props) {
 
   return (
     <div className="flex flex-col h-full min-h-0">
+      {!isMobile && (
         <div className="flex items-center gap-2 border-b border-border bg-muted/20 px-3 py-1.5 shrink-0">
           <DropdownMenu>
             <DropdownMenuTrigger asChild>
@@ -132,7 +133,7 @@ export function Workspace({ sessionId, projectId }: Props) {
                 type="button"
                 disabled={panes.length >= MAX_PANES}
                 className={cn(
-                'flex items-center gap-1 text-xs px-2 py-1 rounded hover:bg-muted max-md:min-h-[44px] max-md:px-3',
+                  'flex items-center gap-1 text-xs px-2 py-1 rounded hover:bg-muted',
                   panes.length >= MAX_PANES && 'opacity-40 cursor-not-allowed hover:bg-transparent'
                 )}
               >
@@ -153,6 +154,7 @@ export function Workspace({ sessionId, projectId }: Props) {
             </DropdownMenuContent>
           </DropdownMenu>
         </div>
+      )}
 
       {isMobile && panes.length > 1 && (
         <div className="flex items-center gap-1 overflow-x-auto border-b border-border bg-muted/10 px-2 py-1 shrink-0">