refactor(sidebar): highlight expanded folder and add per-folder new-conversation button

- Replace the branch badge with a compact count badge; recolor the
  folder name and badge to sidebar-primary on a tinted row background
  when expanded
- Lighten the selected conversation item background so the expanded
  folder row stays the strongest signal
- Add a "+" button on each folder header that reuses a single new-
  conversation tab across folders, disconnecting the old ACP session
  so the connection lifecycle reconnects against the target folder's
  working directory

src/components/conversations/sidebar-conversation-card.tsx

@@ -145,7 +145,7 @@ export const SidebarConversationCard = memo(function SidebarConversationCard({
                 "transition-colors duration-[120ms]",
                 "pr-[0.5rem] pl-7",
                 isSelected
-                  ? "bg-sidebar-primary/15"
+                  ? "bg-sidebar-primary/8"
                   : "hover:bg-[color-mix(in_oklab,var(--sidebar-accent),var(--sidebar-foreground)_2%)]"
               )}
             >