i18n: internationalize all admin pages

Add full i18n coverage for the admin section — locale keys, layout nav,
users, videos, and articles pages (list, new, edit).

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

packages/frontend/src/routes/admin/+layout.svelte

@@ -1,13 +1,14 @@
 <script lang="ts">
   import { page } from "$app/state";
+  import { _ } from "svelte-i18n";
 
   const { children } = $props();
 
-  const navLinks = [
-    { name: "Users", href: "/admin/users", icon: "icon-[ri--team-line]" },
-    { name: "Videos", href: "/admin/videos", icon: "icon-[ri--film-line]" },
-    { name: "Articles", href: "/admin/articles", icon: "icon-[ri--article-line]" },
-  ];
+  const navLinks = $derived([
+    { name: $_("admin.nav.users"), href: "/admin/users", icon: "icon-[ri--team-line]" },
+    { name: $_("admin.nav.videos"), href: "/admin/videos", icon: "icon-[ri--film-line]" },
+    { name: $_("admin.nav.articles"), href: "/admin/articles", icon: "icon-[ri--article-line]" },
+  ]);
 
   function isActive(href: string) {
     return page.url.pathname.startsWith(href);
@@ -20,7 +21,7 @@
     <!-- Mobile top nav -->
     <div class="lg:hidden flex items-center gap-2 py-3 border-b border-border/40">
       <a href="/" class="text-xs text-muted-foreground hover:text-foreground transition-colors shrink-0 mr-2">
-        ← Back
+        {$_("admin.nav.back_mobile")}
       </a>
       {#each navLinks as link (link.href)}
         <a
@@ -43,9 +44,9 @@
       <aside class="hidden lg:flex w-56 shrink-0 flex-col border-r border-border/40">
         <div class="px-4 py-5 border-b border-border/40">
           <a href="/" class="text-xs text-muted-foreground hover:text-foreground transition-colors">
-            ← Back to site
+            {$_("admin.nav.back_to_site")}
           </a>
-          <h1 class="mt-2 text-base font-bold text-foreground">Admin</h1>
+          <h1 class="mt-2 text-base font-bold text-foreground">{$_("admin.nav.title")}</h1>
         </div>
 
         <nav class="flex-1 p-3 space-y-1">