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).

single.php

@@ -42,7 +42,7 @@ while (have_posts()) :
 		</div>
 	<?php endif; ?>
 
-	<main role="main" class="container<?php echo $show_aside ? '' : ' container--no-sidebar'; ?>">
+	<main role="main" class="container">
 		<div class="content-wrap">
 			<div class="content-layout">
 				<?php if (is_active_sidebar('single_top_content')) : ?>