Load a conversation's history on the client

The conversation pages rendered their messages on the server, so a page
the back button restored from the back-forward cache showed an old
thread. The server now renders everything else (rail, header, composer)
and the history (active session, latest messages, active task) loads
from api/admin/bots/conversation-history once the page is on screen,
with a loading line under the header, and again on a pageshow restore.
Messages sent or pushed while it loads are merged with it.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/styles/main.css

@@ -184,6 +184,19 @@
     /* The rail's full (named) width: below it the rail shows icons only.
        A container query, so the mobile drawer opens full too. */
     --container-rail: 220px;
+
+    /* The thin line under a pane header while its page loads
+       (`LoadingLine`): a short bar sweeping across, again and again. */
+    --animate-loading-line: loading-line 1.2s ease-in-out infinite;
+
+    @keyframes loading-line {
+        0% {
+            transform: translateX(-100%);
+        }
+        100% {
+            transform: translateX(300%);
+        }
+    }
 }
 
 body {