thread_view: Adjust empty state and error displays (zed-industries#36774)

Also changes the message editor placeholder depending on the agent.

Release Notes:

- N/A

---------

Co-authored-by: Conrad Irwin <[email protected]>

Author: 2 people

crates/agent2/src/native_agent_server.rs

@@ -23,11 +23,11 @@ impl NativeAgentServer {
 
 impl AgentServer for NativeAgentServer {
     fn name(&self) -> &'static str {
-        "Native Agent"
+        "Zed Agent"
     }
 
     fn empty_state_headline(&self) -> &'static str {
-        "Welcome to the Agent Panel"
+        self.name()
     }
 
     fn empty_state_message(&self) -> &'static str {