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).

inc/helpers/aksrt-archive.php

@@ -122,19 +122,13 @@ if (!function_exists('aksrt_archive_has_sidebar')) :
 	/**
 	 * Resolve whether a stream page should show an aside.
 	 *
-	 * @param string $preferred Preferred sidebar id (compat only).
-	 * @param bool   $enabled   Page-level sidebar toggle.
+	 * @param bool $enabled Page-level sidebar toggle.
 	 */
-	function aksrt_archive_has_sidebar(string $preferred = '', bool $enabled = true): bool {
-		// $preferred kept for call-site compat; visibility is settings/meta driven (legacy).
-		unset($preferred);
+	function aksrt_archive_has_sidebar(bool $enabled = true): bool {
 		if (!$enabled) {
 			return false;
 		}
-		if (function_exists('aksrt_is_show_sidebar')) {
-			return aksrt_is_show_sidebar();
-		}
-		return !wp_is_mobile();
+		return function_exists('aksrt_is_show_sidebar') ? aksrt_is_show_sidebar() : false;
 	}
 endif;