fix: move sidebar arrow hint to top-right instead of vertically centered

The arrow was centered vertically which put it behind the feature cards.
Now positioned at top: 80px where there's open space and it's more visible.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

browse/src/welcome.html

@@ -42,9 +42,8 @@
   /* Sidebar prompt — points RIGHT toward where sidebar opens */
   .sidebar-prompt {
     position: fixed;
-    top: 50%;
+    top: 80px;
     right: 20px;
-    transform: translateY(-50%);
     z-index: 100;
     display: flex;
     align-items: center;