Fix chat scroll: cap the shell at the viewport on /asistente

The shell revamp left SidebarInset unbounded (wrapper is min-h-svh), so
Asistente's flex chain resolved to content height — the chat's internal
scroller never engaged and long answers hid behind the composer.
Asistente now gets h-svh + overflow-hidden on the inset; every other
page keeps body scroll. Verified: inset == viewport on /asistente,
body scroll intact elsewhere.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/components/Layout.tsx

@@ -27,11 +27,14 @@ export default function Layout() {
   const { privacyMode, togglePrivacy } = usePrivacy();
   const { pathname } = useLocation();
   const title = titleFor(pathname);
+  // Asistente hosts its own scroll container (the chat), so its route caps
+  // the shell at the viewport; every other page keeps normal body scroll.
+  const fullHeight = pathname === "/asistente";
 
   return (
     <SidebarProvider>
       <AppSidebar />
-      <SidebarInset>
+      <SidebarInset className={fullHeight ? "h-svh overflow-hidden" : undefined}>
         <header className="sticky top-0 z-40 flex h-14 shrink-0 items-center gap-2 border-b border-border bg-background/90 px-4 backdrop-blur-sm">
           <SidebarTrigger className="-ml-1" />
           <Separator