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/static/css/app.css

@@ -626,7 +626,9 @@ button, input, textarea, select {
     z-index: 40;
     box-shadow: var(--shadow-lg);
   }
-  .sidebar[data-collapsed="true"] { display: none; }
+  /* Hiding it is the `hidden` attribute, forced to win at the top of this
+     file. There used to be a `[data-collapsed="true"]` rule here that nothing
+     ever set. */
 }
 
 /* --- Toasts ----------------------------------------------------------------