fix(web): uniform category spacing in channel sidebar

The scroll container's space-y-[2px] utility set margin-bottom:0 (via
Tailwind's space-y reverse mechanism) on every category wrapper except
the first, with higher specificity (.space-y > :not ~ :not = 0,3,0) than
each category's mb-[19px] (0,1,0). This zeroed the 19px separator for all
but the first category, so the inter-category gap rendered only below the
first category and shifted when categories were reordered.

Remove the redundant container-level space-y; the per-category mb-[19px]
is the intended separator and now applies uniformly.

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

@@ -581,7 +581,7 @@ export function ChannelSidebar() {
       </div>
 
       {/* Channels — dynamic category layout */}
-      <div ref={scrollContainerRef} className="flex-1 overflow-y-auto pt-3 px-2 space-y-[2px] no-scrollbar" style={{ paddingBottom: floatingPanelHeight + 24 }} onDrop={containerHandlers.onDrop} onDragOver={containerHandlers.onDragOver} onContextMenu={handleSidebarContextMenu}>
+      <div ref={scrollContainerRef} className="flex-1 overflow-y-auto pt-3 px-2 no-scrollbar" style={{ paddingBottom: floatingPanelHeight + 24 }} onDrop={containerHandlers.onDrop} onDragOver={containerHandlers.onDragOver} onContextMenu={handleSidebarContextMenu}>
         {showChannelSkeleton ? (
           <div className="px-2 pt-3" role="status" aria-label="Loading channels">
             {/* Category group 1 */}