thread_view: Add recent history entries & adjust empty state (#36625)

Release Notes:

- N/A

crates/agent2/src/history_store.rs

@@ -345,4 +345,8 @@ impl HistoryStore {
             .retain(|old_entry| old_entry != entry);
         self.save_recently_opened_entries(cx);
     }
+
+    pub fn recent_entries(&self, limit: usize, cx: &mut Context<Self>) -> Vec<HistoryEntry> {
+        self.entries(cx).into_iter().take(limit).collect()
+    }
 }

crates/agent2/src/native_agent_server.rs

@@ -27,7 +27,7 @@ impl AgentServer for NativeAgentServer {
     }
 
     fn empty_state_headline(&self) -> &'static str {
-        ""
+        "Welcome to the Agent Panel"
     }
 
     fn empty_state_message(&self) -> &'static str {