feat: style scrollbars globally using theme colors

Thin scrollbars using --border for thumb and transparent track,
with --muted-foreground on hover. Uses both scrollbar-color (Firefox)
and ::-webkit-scrollbar (Chrome/Safari).

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

packages/frontend/src/app.css

@@ -104,6 +104,26 @@
   * {
     border-color: var(--border);
     outline-color: var(--ring);
+    scrollbar-width: thin;
+    scrollbar-color: var(--border) transparent;
+  }
+
+  *::-webkit-scrollbar {
+    width: 6px;
+    height: 6px;
+  }
+
+  *::-webkit-scrollbar-track {
+    background: transparent;
+  }
+
+  *::-webkit-scrollbar-thumb {
+    background-color: var(--border);
+    border-radius: 9999px;
+  }
+
+  *::-webkit-scrollbar-thumb:hover {
+    background-color: var(--muted-foreground);
   }
 
   .prose h2 {