feat: add article management (list, edit, delete)

- Add admin article list page at /admin/articles with status badges,
  pin marker, publish time, and edit/delete actions
- Share the create/edit form: dynamic action URL, prefilled fields,
  is_top checkbox state, and create-vs-edit heading
- Add ArticleUpdate/ArticleDelete handlers (soft-delete via DeletedAt);
  stamp PublishedAt on first publish
- Add "Manage Articles" entry on the dashboard
- Hide home page sign-in/dashboard buttons
- Add EN/ZH i18n keys for the management UI

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

templates/admin/article_create.html

@@ -2,7 +2,7 @@
 {{template "header" .}}
 
 <section class="max-w-3xl mx-auto px-4 py-10">
-    <h2 class="text-2xl font-bold text-gray-900 mb-8">{{index .Tr "article_create_title"}}</h2>
+    <h2 class="text-2xl font-bold text-gray-900 mb-8">{{.FormTitleText}}</h2>
 
     {{if .Error}}
     <div class="bg-red-50 border border-red-200 text-red-700 px-4 py-3 rounded-lg mb-6 text-sm">
@@ -10,7 +10,7 @@
     </div>
     {{end}}
 
-    <form action="/admin/articles/new" method="post" class="space-y-6">
+    <form action="{{.FormAction}}" method="post" class="space-y-6">
         <!-- Title -->
         <div>
             <label class="block text-sm font-medium text-gray-700 mb-1">{{index .Tr "article_title"}}</label>
@@ -52,7 +52,7 @@
 
         <!-- IsTop -->
         <div class="flex items-center gap-2">
-            <input type="checkbox" name="is_top" value="1" id="isTopCheckbox"
+            <input type="checkbox" name="is_top" value="1" id="isTopCheckbox" {{if .FormIsTop}}checked{{end}}
                 class="w-4 h-4 text-blue-600 border-gray-300 rounded focus:ring-blue-500">
             <label for="isTopCheckbox" class="text-sm font-medium text-gray-700">{{index .Tr "article_is_top"}}</label>
         </div>