fix: sidebar sticky on desktop — user card always visible

Add position: sticky, top: 0, height: 100dvh to aside on desktop.
Sidebar now stays fixed to viewport height, nav scrolls internally,
user card is always visible at the bottom without scrolling.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/Layout.jsx

@@ -56,7 +56,9 @@ export default function Layout() {
         ...(isMobile ? {
           position: 'fixed', top: 0, left: sidebarOpen ? 0 : -220,
           height: '100dvh', zIndex: 300, transition: 'left 0.25s ease',
-        } : {}),
+        } : {
+          position: 'sticky', top: 0, height: '100dvh', overflowY: 'auto',
+        }),
       }}>
         {/* Logo */}
         <div style={{ padding: '1.5rem 1.25rem 1.25rem', borderBottom: '1px solid var(--border)' }}>