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/chat/Message.tsx

@@ -243,7 +243,7 @@ export function Message({ message, isCompact, isFirstInGroup }: MessageProps) {
 
             {/* Reactions */}
             {Object.keys(reactionGroups).length > 0 && (
-              <div className="flex flex-wrap gap-1 mt-1">
+              <div className="flex flex-wrap gap-1">
                 {Object.entries(reactionGroups).map(([emoji, { count, me }]) => (
                   <button
                     key={emoji}