feat: centralize breadcrumbs in topbar and remove history navigation

Move breadcrumb rendering from individual page templates into the core
topbar. Each page now sets $breadcrumbs in its action .php file; the
topbar renders it automatically via the shared partial.

- Remove global back/forward buttons and app-nav-history.js component
- Remove Alt+Arrow keyboard shortcuts for history navigation
- Render breadcrumb in topbar-left section (replaces button area)
- Clean up breadcrumb CSS: context-neutral base (flex, no margin)
- Recalculate sticky titlebar offset in details container
- Migrate all 41 pages (core + helpdesk, audit, addressbook, api-docs)
- Add missing breadcrumbs to addressbook detail view
- Update architecture contract tests (nav-history references removed)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pages/admin/users/create(default).phtml

@@ -12,12 +12,6 @@
 
   <section>
     <?php
-    $breadcrumbs = [
-      ['label' => t('Home'), 'path' => 'admin'],
-      ['label' => t('Users'), 'path' => 'admin/users'],
-      ['label' => t('Create user')],
-    ];
-    require templatePath('partials/app-breadcrumb.phtml');
     $titlebar = [
       'title' => t('Create user'),
       'backHref' => 'admin/users',