fix(sidebar): collapse empty sidebar column on every scene

Sidebar visibility had two disconnected decisions: the body site-layout-*
class (via aksrt_is_show_sidebar_filter) only checked widget activity for
singles, while the homepage aside renderer skipped the column and
sidebar.php rendered an empty one. With the toggle on and no widgets, the
homepage kept a reserved gutter and squeezed the main content aside.

- collapse every scene to no-sidebar when the sidebar widget stack is empty
- resolve home_sidebar before the page branch so a static front page is
  checked against the zones the homepage renders
- guard the homepage aside with aksrt_is_show_sidebar() alone and drop the
  duplicate aksrt_homepage_has_sidebar gate
- early-return sidebar.php instead of rendering an empty column
- drop dead code: archive helper $preferred param, wp_is_mobile fallback,
  container--no-sidebar class, unreachable front-page fallback
- update post-visual-contract to pin the single visibility decision

Verification: php -l on all changed files; node --test
tests/widget-system/*.test.cjs 242/243 (the toast-contract failure is
pre-existing and unrelated).

front-page.php

@@ -11,21 +11,6 @@
 
 get_header();
 
-if (function_exists('aksrt_homepage_render_layout')) {
-	aksrt_homepage_render_layout();
-} else {
-	?>
-	<main id="main-content" class="site-main content-shell" tabindex="-1">
-		<div class="content-layout content-layout--no-sidebar">
-			<div class="content-layout__primary">
-				<section class="widget widget-card box-body">
-					<h1 class="entry-title"><?php esc_html_e('首页骨架未加载', 'aksrt'); ?></h1>
-					<p><?php esc_html_e('固定首页骨架当前不可用，请检查主题加载链路。', 'aksrt'); ?></p>
-				</section>
-			</div>
-		</div>
-	</main>
-	<?php
-}
+aksrt_homepage_render_layout();
 
 get_footer();