One thread template, one sidebar toggle

chat/index.html and chat/_thread.html held the same loop, so anything added
to the conversation -- a compaction divider, say -- would have had to be
written into both and kept in step by hand. index.html includes the partial
instead.

The sidebar toggle was a raw inline onclick, the only one left in the
application. app.js already delegates [data-toggle="#selector"] and gives
open/close, aria-expanded and an is-active button state for free; the chat
settings gear has used it all along. Also deletes the
.sidebar[data-collapsed="true"] rule, which nothing has ever set.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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

@@ -1,7 +1,11 @@
 {#
   The whole thread. Returned after a rewind, which changes an arbitrary number
   of messages at once -- replacing the lot is simpler and less error-prone than
-  working out which individual bubbles to remove.
+  working out which individual bubbles to remove. Also included by
+  chat/index.html, so the conversation is described in exactly one place.
+
+  Deliberately has no root element: it is swapped with innerHTML into #thread,
+  and an outerHTML swap would take the container with it.
 #}
 {% for message in messages %}
   {% with body_html = bodies.get(message.id, "") %}