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/ChannelSidebar.tsx

@@ -119,8 +119,8 @@ export function ChannelSidebar() {
   if (!server) {
     return (
       <>
-      <div className="w-60 md:w-full bg-surface-channel flex flex-col flex-shrink-0 select-none md:pl-[72px]">
-        <div className="h-12 px-[10px] flex items-center shadow-header z-10">
+      <div className="w-60 md:w-full bg-surface-channel flex flex-col flex-shrink-0 select-none md:pl-[72px] border-r border-border-hard">
+        <div className="h-12 px-[10px] flex items-center border-b border-border-hard z-10">
           <button className="flex-1 bg-surface-base text-txt-tertiary text-[13px] font-medium py-[5px] px-2 rounded-[4px] text-left hover:bg-surface-base/80 transition-colors">
             Find or start a conversation
           </button>
@@ -271,9 +271,9 @@ export function ChannelSidebar() {
 
   return (
     <>
-    <div className="w-60 md:w-full bg-surface-channel flex flex-col flex-shrink-0 select-none md:pl-[72px]">
+    <div className="w-60 md:w-full bg-surface-channel flex flex-col flex-shrink-0 select-none md:pl-[72px] border-r border-border-hard">
       {/* Server header */}
-      <div className="h-12 flex items-center shadow-header z-10 group/header">
+      <div className="h-12 flex items-center border-b border-border-hard z-10 group/header">
         <button
           onClick={() => openModal('serverSettings')}
           className="flex-1 h-full px-4 flex items-center justify-between hover:bg-interactive-hover transition-colors min-w-0"