feat: unify page layout with consistent page titles and structure

- Add page title (h1) to all main pages for consistent hierarchy
  - Wrap page content in container/content div structure
  - Handle 404 error for unsupported OAuth excluded models API
  - Add cache price input field in usage page model pricing
  - Add upgrade required i18n messages for older CPA versions
  - Import mixins in page-level SCSS modules

src/pages/Settings/Settings.module.scss

@@ -1,3 +1,5 @@
+@use '../../styles/mixins' as *;
+
 .container {
   width: 100%;
 }