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>

main.go

@@ -57,8 +57,12 @@ func main() {
 	admin.Use(middleware.AuthRequired())
 	{
 		admin.GET("", handlers.AdminDashboard(db))
+		admin.GET("/articles", handlers.ArticleListPage(db))
 		admin.GET("/articles/new", handlers.ArticleCreatePage(db))
 		admin.POST("/articles/new", handlers.ArticleCreate(db))
+		admin.GET("/articles/:id/edit", handlers.ArticleEditPage(db))
+		admin.POST("/articles/:id/edit", handlers.ArticleUpdate(db))
+		admin.POST("/articles/:id/delete", handlers.ArticleDelete(db))
 	}
 
 	// Protected profile routes.