refactor: align article author with VideoModel, streamline selects, fix flyout inert

- Remove ArticleAuthor type; article.author now reuses VideoModel (id, artist_name, slug, avatar)
- updateArticle accepts authorId; author selectable in admin article edit page
- Article edit: single Select with bind:value + $derived selectedAuthor display
- Video edit: replace pill toggles with Select type="multiple" bind:value for models
- Video table: replace inline badge spans with Badge component
- Magazine: display artist_name throughout, author bio links to model profile
- Fix flyout aria-hidden warning: replace with inert attribute

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

packages/frontend/src/lib/components/header/header.svelte

@@ -172,7 +172,7 @@
 <!-- Flyout panel -->
 <div
   class={`fixed inset-y-0 left-0 z-50 w-80 max-w-[85vw] bg-card/95 backdrop-blur-xl shadow-2xl shadow-primary/20 border-r border-border/30 transform transition-transform duration-300 ease-in-out lg:hidden overflow-y-auto flex flex-col ${isMobileMenuOpen ? "translate-x-0" : "-translate-x-full"}`}
-  aria-hidden={!isMobileMenuOpen}
+  inert={!isMobileMenuOpen || undefined}
 >
   <!-- Panel header -->
   <div class="flex items-center px-5 h-16 shrink-0 border-b border-border/30">