fix: switching profiles mid-conversation starts a new session instead of cross-tagging

A session with messages belongs to the profile it was created under. Switching
profiles while a conversation is in progress should not retag that session or
update its workspace/model in place — that would corrupt the session's context.

New behavior:
- Session has NO messages (empty): profile switch updates it in place (model,
  workspace). Works exactly as before — nothing was started yet.
- Session HAS messages (in progress): profile switch calls newSession() to
  start a fresh session tagged to the new profile. The old session is left
  untouched. Toast: 'Switched to profile: X — new conversation started'.
- Agent busy: blocked as before, no change.

Also: S._profileDefaultWorkspace is now consumed (set to null) inside
newSession() after the first use, so it doesn't keep forcing the same
workspace on every subsequent new session after a switch.

static/sessions.js

@@ -13,9 +13,10 @@ async function newSession(flash){
   MSG_QUEUE.length=0;updateQueueBadge();
   S.toolCalls=[];
   clearLiveToolCards();
-  // Use profile default workspace for new sessions after a profile switch,
+  // Use profile default workspace for new sessions after a profile switch (one-shot),
   // otherwise inherit from the current session (or let server pick the default)
-  const inheritWs=S._profileDefaultWorkspace||( S.session?S.session.workspace:null);
+  const inheritWs=S._profileDefaultWorkspace||(S.session?S.session.workspace:null);
+  S._profileDefaultWorkspace=null; // consume — only applies to the first new session after switch
   const data=await api('/api/session/new',{method:'POST',body:JSON.stringify({model:$('modelSelect').value,workspace:inheritWs})});
   S.session=data.session;S.messages=data.session.messages||[];
   if(flash)S.session._flash=true;