fix: reset _messagesTruncated flag on new session creation

newSession() did not reset _messagesTruncated or _oldestIdx, unlike
loadSession() which resets both at line 590. When a user switched from
a long session (messages > _INITIAL_MSG_LIMIT) to a new session, the
stale _messagesTruncated=true caused renderMessages() to show the
'Scroll up or click to load older messages' indicator on a fresh
conversation with only 1 message.

Add the same reset that loadSession() already performs so newSession()
starts with clean pagination state.

static/sessions.js

@@ -455,6 +455,8 @@ async function newSession(flash, options={}){
   _newSessionInFlight=(async()=>{
     updateQueueBadge();
     S.toolCalls=[];
+    _messagesTruncated=false;
+    _oldestIdx=0;
     clearLiveToolCards();
     // One-shot profile-switch workspace: applied to the first new session after a profile
     // switch, then cleared.  Use a dedicated flag so S._profileDefaultWorkspace (the