Fix timeline top arrow not mirroring bottom arrow position

Reduce arrow button font-size and height from 8px to 6px and set
line-height to 0 so both arrows sit symmetrically tight against the
marker.

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

js/snapshot_manager.js

@@ -2083,16 +2083,16 @@ const CSS = `
     background: none;
     border: none;
     color: #3b82f6;
-    font-size: 8px;
+    font-size: 6px;
     cursor: pointer;
     padding: 0;
     margin: 0;
-    line-height: 1;
+    line-height: 0;
     display: flex;
     align-items: center;
     justify-content: center;
     width: 18px;
-    height: 8px;
+    height: 6px;
     border-radius: 2px;
     flex-shrink: 0;
     opacity: 0.7;