fix: panel separator borders and reaction spacing to match prototype

Replace @apply border-none with @apply border-0 on global * selector.
border-none sets border-style: none, which silently kills all Tailwind
border utilities (they only set width/color, relying on preflight's
border-style: solid). border-0 sets border-width: 0 instead, preserving
the solid style so border-r/border-l/border-b utilities render correctly.

- Add border-r border-border-hard to ChannelSidebar (both DM and server views)
- Add border-l border-border-hard to MemberSidebar and ActivityPanel
- Replace shadow-header with border-b border-border-hard on sidebar headers
- Remove duplicate mt-1 on reaction container (parent gap-1 already provides 4px)

packages/web/src/components/layout/ActivityPanel.tsx

@@ -68,7 +68,7 @@ export function ActivityPanel() {
   );
 
   return (
-    <div className="w-60 bg-surface-channel flex-shrink-0 overflow-y-auto select-none no-scrollbar hidden md:block">
+    <div className="w-60 bg-surface-channel flex-shrink-0 overflow-y-auto select-none no-scrollbar hidden md:block border-l border-border-hard">
       <div className="p-3">
         <h3 className="text-[20px] font-bold text-txt-primary mb-4 px-2">Active Now</h3>