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/routes/admin/videos/+page.svelte

@@ -65,7 +65,7 @@
 <Meta title={$_("admin.videos.title")} description={null} />
 
 <div class="py-3 sm:py-6 lg:pl-6">
-  <div class="flex items-center justify-between mb-6 px-3 sm:px-0">
+  <div class="flex items-center justify-between mb-6">
     <h1 class="text-2xl font-bold">{$_("admin.videos.title")}</h1>
     <div class="flex items-center gap-3">
       <span class="text-sm text-muted-foreground"
@@ -81,7 +81,7 @@
   </div>
 
   <!-- Filters -->
-  <div class="flex flex-wrap items-center gap-3 mb-4 px-3 sm:px-0">
+  <div class="flex flex-wrap items-center gap-3 mb-4">
     <Input
       placeholder={$_("admin.videos.search_placeholder")}
       class="max-w-xs"
@@ -209,7 +209,7 @@
 
   <!-- Pagination -->
   {#if data.total > data.limit}
-    <div class="flex items-center justify-between mt-4 px-3 sm:px-0">
+    <div class="flex items-center justify-between mt-4">
       <span class="text-sm text-muted-foreground">
         {$_("admin.users.showing", {
           values: {