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).

index.php

@@ -21,8 +21,8 @@ get_header();
 
 <?php
 // legacy dual-column: sidebar is a sibling of .content-wrap under main.container.
-// Settings and page metadata own visibility; widget activity never changes
-// the body layout after render.
+// aksrt_is_show_sidebar() is the single visibility decision (settings, post/term
+// meta, and widget activity); the body site-layout-* class carries the geometry.
 $has_sidebar = function_exists('aksrt_is_show_sidebar') ? aksrt_is_show_sidebar() : true;
 ?>
 <main id="main-content" class="site-main container" tabindex="-1">