fix(desktop): inset sticky human messages with --sticky-human-top

Pin user bubbles 0.75rem below the scroll top via a single token instead of
flush top-0, so the sticky header doesn't sit hard against the thread edge.

apps/desktop/src/styles.css

@@ -272,6 +272,7 @@
     --conversation-line-height: 1.125rem;
     --conversation-caption-line-height: 1rem;
     --conversation-turn-gap: 0.375rem;
+    --sticky-human-top: 0.75rem;
     --file-tree-row-height: 1.375rem;
 
     --composer-width: 48.75rem;
@@ -704,6 +705,10 @@ canvas {
   padding-inline-start: var(--md-text-indent, 0.5rem);
 }
 
+[data-slot='aui_user-message-root'] {
+  top: var(--sticky-human-top);
+}
+
 [data-slot='aui_user-message-root'],
 [data-slot='aui_edit-composer-root'] {
   font-size: var(--conversation-text-font-size);