style: refine admin edit forms and fix mobile padding

- Remove back button from admin entity edit pages (sidebar handles navigation)
- Remove cancel button from video/article forms, make submit button full-width
- Show actual entity title + subtitle on video/article edit pages
- Remove asterisks from Title/Slug field labels in i18n
- Remove px-3 sm:px-0 from all admin list page headers/filters (fixes mobile padding)

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

packages/frontend/src/lib/i18n/locales/en.ts

@@ -951,8 +951,8 @@ export default {
       cover_image: "Cover image",
       tags: "Tags",
       publish_date: "Publish date",
-      title_field: "Title *",
-      slug_field: "Slug *",
+      title_field: "Title",
+      slug_field: "Slug",
       title_slug_required: "Title and slug are required",
       image_uploaded: "Image uploaded",
       image_upload_failed: "Image upload failed",