fix(web): flat separators with wrapper divs, first-row border, DM sidebar title case

- FriendItem/RequestItem: add plain wrapper div so divide-y border
  lands on a non-rounded element (fixes curved separator lines)
- Add border-t to divide-y containers so first row also gets top line
- Remove uppercase from "Direct Messages" in DM sidebar

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

@@ -472,7 +472,7 @@ export function ChannelSidebar() {
           </div>
 
           <div className="mt-[18px] px-2 mb-1 flex items-center justify-between group">
-            <span className="text-[12px] font-bold text-txt-tertiary uppercase tracking-wider">Direct Messages</span>
+            <span className="text-[12px] font-bold text-txt-tertiary tracking-wider">Direct Messages</span>
             <button
               onClick={() => openModal('newDm')}
               className="text-txt-tertiary hover:text-txt-primary transition-colors"