feat(mobile): add "New chat" to tab long-press context menu

With the Split button hidden on mobile (G1), users need another path
to create additional chat panes. Add a "New chat" ContextMenuItem at
the top of each tab's context menu, separated from Rename / Close /
etc. by a ContextMenuSeparator. Wired to the existing onNewChat prop
— no plumbing change. Available on both long-press (mobile) and
right-click (desktop).

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

apps/web/src/components/ChatTabBar.tsx

@@ -123,6 +123,10 @@ export function ChatTabBar({
               </div>
             </ContextMenuTrigger>
             <ContextMenuContent>
+              <ContextMenuItem onSelect={() => onNewChat()}>
+                New chat
+              </ContextMenuItem>
+              <ContextMenuSeparator />
               <ContextMenuItem onSelect={() => startRename(chat.id, chat.name)}>
                 Rename
               </ContextMenuItem>