fix: improve App Config slide-in panel layout

- Narrowed panel from 640px to 520px so main table columns stay visible
- Settings grid uses CSS grid (3 columns) for proper wrapping
- Removed unused PanelActions component that caused white footer bar

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

ui/src/pages/Admin/AppConfigPage.module.css

@@ -1,5 +1,5 @@
 .widePanel {
-  width: 640px !important;
+  width: 520px !important;
 }
 
 .panelSection {
@@ -16,9 +16,9 @@
 }
 
 .settingsGrid {
-  display: flex;
-  flex-wrap: wrap;
-  gap: 16px;
+  display: grid;
+  grid-template-columns: 1fr 1fr 1fr;
+  gap: 12px;
 }
 
 .field {