feat(agent): Unselect session when archiving currently selected session

When a session is archived and it was the currently selected session,
it is now automatically deselected and the empty state placeholder is
shown in the right panel.

- Updated SessionManagerProps to allow null for onSelectSession callback
- Added logic to deselect session on archive if it was selected
- Added data-testid attributes for testing

app/src/components/session-manager.tsx

@@ -47,7 +47,7 @@ function generateRandomSessionName(): string {
 
 interface SessionManagerProps {
   currentSessionId: string | null;
-  onSelectSession: (sessionId: string) => void;
+  onSelectSession: (sessionId: string | null) => void;
   projectPath: string;
   isCurrentSessionThinking?: boolean;
 }
@@ -141,6 +141,10 @@ export function SessionManager({
 
     const result = await window.electronAPI.sessions.archive(sessionId);
     if (result.success) {
+      // If the archived session was currently selected, deselect it
+      if (currentSessionId === sessionId) {
+        onSelectSession(null);
+      }
       await loadSessions();
     }
   };
@@ -357,6 +361,7 @@ export function SessionManager({
                     variant="ghost"
                     onClick={() => handleArchiveSession(session.id)}
                     className="h-7 w-7 p-0"
+                    data-testid={`archive-session-${session.id}`}
                   >
                     <Archive className="w-3 h-3" />
                   </Button>

app/src/components/views/agent-view.tsx

@@ -471,7 +471,7 @@ export function AgentView() {
 
         {/* Messages */}
         {!currentSessionId ? (
-          <div className="flex-1 flex items-center justify-center">
+          <div className="flex-1 flex items-center justify-center" data-testid="no-session-placeholder">
             <div className="text-center">
               <Bot className="w-12 h-12 text-muted-foreground mx-auto mb-4 opacity-50" />
               <h2 className="text-lg font-semibold mb-2">