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>

src/lembas/web/templates/chat/index.html

@@ -65,11 +65,12 @@
       <div class="topbar__actions">
         {#
           A link, not a script: the flag lives in the URL, so it survives a
-          reload and can be bookmarked.
+          reload and can be bookmarked. The URL is built in `chat_index` so it
+          keeps the chosen model, folder and kind.
         #}
         {% if not chat and can.get("chat.create") %}
         <a class="btn btn--icon {{ 'is-active' if starting_temporary }}"
-           href="{{ '/chat' if starting_temporary else '/chat?temporary=1' }}"
+           href="{{ temporary_toggle_url }}"
            aria-label="{{ t('Temporary chat') }}"
            title="{% if starting_temporary %}Starting a temporary chat. Click to go back to a normal one.{% else %}Start a temporary chat: not listed in the sidebar, and removed after a day.{% endif %}">
           {{ icon("clock") }}