feat(canvas): show loading state while agent history loads

Track agent history loading in the workspace and render a spinner with a
localized "loading conversation history" message in the assistant feed,
so the empty state no longer flashes before history resolves.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/ui/styles.css

@@ -3789,6 +3789,24 @@ button:disabled {
   background: #dfe0e2;
 }
 
+.assistant-history-loading {
+  width: 100%;
+  min-height: 100%;
+  flex: 1 0 auto;
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+  gap: 10px;
+  color: #8a9099;
+  font-size: 13px;
+  line-height: 20px;
+}
+
+.assistant-history-loading svg {
+  color: #646a73;
+}
+
 .assistant-scroll-to-end {
   position: absolute;
   inset-inline: 0;