fix(sidebar): restore section padding to 16px top / 10px bottom

12px was too tight against dividers; the original spacing complaint
was the flex:1 empty stretch (fixed separately), not section padding.

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

ui/src/index.css

@@ -300,8 +300,8 @@ textarea:focus-visible {
 .sidebar-section {
   display: flex;
   flex-direction: column;
-  padding-top: 12px;
-  padding-bottom: 4px;
+  padding-top: 16px;
+  padding-bottom: 10px;
   border-top: 1px solid var(--border-strong);
 }