refactor(ui): unify pricing edit modal with FullScreenPanel

Replace Dialog component with FullScreenPanel in PricingEditModal
to match the UI style of other edit dialogs (provider, MCP).

Changes:
- Switch from small centered Dialog to full-screen panel
- Add back button in header and fixed footer for actions
- Add Save/Plus icons to submit button

src/components/usage/PricingConfigPanel.tsx

@@ -202,6 +202,7 @@ export function PricingConfigPanel() {
 
       {editingModel && (
         <PricingEditModal
+          open={!!editingModel}
           model={editingModel}
           isNew={isAddingNew}
           onClose={() => {