A temporary chat that keeps the model it was started on

The new-chat screen keeps its preselections in the query string, and the
three ways off it each rebuilt the URL with only their own key: the
Temporary button went to /chat?temporary=1, the model picker to
/chat?model=<id>, and /temp to /chat?temporary=1. So each undid the other,
and a temporary chat could only be started on the default model. The
folder from "New chat here" and the agent kind were dropped the same way.

chat_index now builds both URLs from one set of carried values and changes
only its own key; /temp keeps location.search when run on /chat.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -16,6 +16,15 @@ for 1.0.0 have something to be assembled from.
 
 ## Unreleased
 
+## 1.9.1
+
+- **A temporary chat can be started on any model.** On the new-chat screen,
+  turning on Temporary switched the model back to the default, and choosing a
+  model switched Temporary off, so a temporary chat could only ever be started
+  on the default model. The Temporary button, the model menu and `/temp` now
+  keep each other's choice, and also keep the folder a chat was started in
+  ("New chat here") and whether it is an agent chat.
+
 ## 1.9.0
 
 The model menu says which model is loaded, and a new chat now matches the