fix(chat): show the timeout message instead of an empty bubble (#6764) (#6768)

_isAgent was a const inside the try block of handleChatSubmit, but the
catch block reads it to pick the timeout message, so a client timeout
threw a ReferenceError and left the reply bubble empty. Declare it next
to the other catch-visible variables and assign it where it is computed.

static/js/chat.js

@@ -2588,6 +2588,7 @@ import { invalidateSettings } from './appConfig.js';
     currentHolder = null;
     
     let abortCtrl = null;
+    let _isAgent = false;  // read by the catch block's timeout message
     let streamingTTS = false;
     try {
       // Re-enable auto-scroll when user sends a message
@@ -2985,7 +2986,7 @@ import { invalidateSettings } from './appConfig.js';
       currentAbort = abortCtrl;
 
 	      const _tState = Storage.loadToggleState();
-	      const _isAgent = (_tState.mode || 'chat') === 'agent' || !!_tState.plan_mode || workspaceAgentIntent;
+	      _isAgent = (_tState.mode || 'chat') === 'agent' || !!_tState.plan_mode || workspaceAgentIntent;
 
       // Timeout: 6 min for research and agent mode, 3 min otherwise
       const timeoutMs = el('research-toggle').checked || _isAgent ? RESEARCH_TIMEOUT_MS : DEFAULT_TIMEOUT_MS;

tests/test_chat_timeout_message_static.py

@@ -0,0 +1,13 @@
+from pathlib import Path
+import re
+
+
+ROOT = Path(__file__).resolve().parents[1]
+
+
+def test_timeout_catch_can_read_is_agent():
+    """The catch block reads _isAgent, so it must not be a try-scoped const (#6764)."""
+    src = (ROOT / "static/js/chat.js").read_text()
+    assert re.search(r"^\s*let _isAgent = false;", src, re.M)
+    assert not re.search(r"\bconst _isAgent\b", src)
+    assert re.search(r"^\s*_isAgent = \(_tState\.mode", src, re.M)