Push arrow glyphs to edges of timeline bar

Top arrow aligns content to flex-start (top edge), bottom arrow to
flex-end (bottom edge), so both arrow tips sit at their respective
edges instead of centering in the available space.

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

js/snapshot_manager.js

@@ -2091,7 +2091,7 @@ const CSS = `
     margin: 0;
     line-height: 1;
     display: flex;
-    align-items: center;
+    align-items: flex-start;
     justify-content: center;
     width: 18px;
     flex: 1;
@@ -2100,6 +2100,9 @@ const CSS = `
     opacity: 0.7;
     transition: opacity 0.1s, background 0.1s;
 }
+.snap-timeline-branch-btn:last-child {
+    align-items: flex-end;
+}
 .snap-timeline-branch-btn:hover {
     opacity: 1;
     background: rgba(59, 130, 246, 0.2);