Fix top arrow to mirror bottom arrow style

Restore 8px size for both arrows. Top arrow uses align-items:flex-end
to push glyph down against marker, bottom arrow uses flex-start to
push glyph up against marker, making them symmetrical.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

js/snapshot_manager.js

@@ -2083,21 +2083,24 @@ const CSS = `
     background: none;
     border: none;
     color: #3b82f6;
-    font-size: 6px;
+    font-size: 8px;
     cursor: pointer;
     padding: 0;
     margin: 0;
-    line-height: 0;
+    line-height: 1;
     display: flex;
-    align-items: center;
+    align-items: flex-end;
     justify-content: center;
     width: 18px;
-    height: 6px;
+    height: 8px;
     border-radius: 2px;
     flex-shrink: 0;
     opacity: 0.7;
     transition: opacity 0.1s, background 0.1s;
 }
+.snap-timeline-branch-btn:last-child {
+    align-items: flex-start;
+}
 .snap-timeline-branch-btn:hover {
     opacity: 1;
     background: rgba(59, 130, 246, 0.2);