style(layout): sidebar stripe + topbar gradient + bell spring + search ring

Sidebar active items: 4px brand left-edge stripe (rounded-r-full) replacing the
border-l-2 + bg shift; section header smaller-caps + brand-200 colour; user-footer
avatar gets shadow-sm + ring-2 ring-white/30.

Topbar '+ New' uses bg-gradient-brand with shadow-sm + scale-1.02 hover. User
avatar trigger gets shadow-sm + ring-2 ring-white. Notification badge gets
gradient-brand fill + ring-2 ring-background + animate-badge-pop spring keyframe
(retriggers on count change via key={unreadCount}). Command search gets shadow-xs
inset + brand focus ring (ring-4 ring-brand/15).

Adds badge-pop keyframes to tailwind config.

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

tailwind.config.ts

@@ -163,10 +163,16 @@ export default {
             height: '0',
           },
         },
+        'badge-pop': {
+          '0%': { transform: 'scale(0.5)', opacity: '0' },
+          '60%': { transform: 'scale(1.18)', opacity: '1' },
+          '100%': { transform: 'scale(1)', opacity: '1' },
+        },
       },
       animation: {
         'accordion-down': 'accordion-down 0.2s ease-out',
         'accordion-up': 'accordion-up 0.2s ease-out',
+        'badge-pop': 'badge-pop 0.32s cubic-bezier(0.34, 1.56, 0.64, 1)',
       },
     },
   },