feat(sidebar): restructure layout and fix agent selection highlighting

- Move New Agent button to top header, Settings button to bottom footer
- Settings screen now uses MenuHeader for consistent hamburger menu
- Remove default background from agent list items (only show on hover/selected)
- Fix selected agent highlighting by parsing serverId:agentId from pathname
  instead of using useLocalSearchParams (which doesn't update on same-pattern route navigation)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

packages/app/src/components/agent-list.tsx

@@ -57,9 +57,12 @@ export function AgentList({ agents, isRefreshing = false, onRefresh, selectedAge
         to: "agent",
         params: { serverId, agentId },
       });
+
       const shouldReplace = pathname.startsWith("/agent/");
       const navigate = shouldReplace ? router.replace : router.push;
+
       onAgentSelect?.();
+
       navigate({
         pathname: "/agent/[serverId]/[agentId]",
         params: {
@@ -91,7 +94,8 @@ export function AgentList({ agents, isRefreshing = false, onRefresh, selectedAge
     ({ item: agent }) => {
       const timeAgo = formatTimeAgo(agent.lastActivityAt);
       const isRunning = agent.status === "running";
-      const isSelected = selectedAgentId === agent.id;
+      const agentKey = `${agent.serverId}:${agent.id}`;
+      const isSelected = selectedAgentId === agentKey;
       const statusColor = isRunning ? "#3b82f6" : agent.requiresAttention ? "#22c55e" : null;
 
       return (
@@ -225,7 +229,6 @@ const styles = StyleSheet.create((theme) => ({
     paddingHorizontal: theme.spacing[3],
     borderRadius: theme.borderRadius.lg,
     marginBottom: theme.spacing[2],
-    backgroundColor: theme.colors.palette.zinc[950],
   },
   agentItemSelected: {
     backgroundColor: theme.colors.palette.zinc[800],