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/search/index().php

@@ -29,6 +29,11 @@ $csrfKey = Session::$csrfSessionKey;
 $csrfToken = $session[$csrfKey] ?? '';
 
 Buffer::set('title', t('Search results'));
+
+$breadcrumbs = [
+    ['label' => t('Home'), 'path' => 'admin'],
+    ['label' => t('Search')],
+];
 Buffer::set('grid_lang', json_encode(gridLang(), JSON_UNESCAPED_UNICODE | JSON_UNESCAPED_SLASHES));
 Buffer::set(
     'grid_csrf',

pages/search/index(default).phtml

@@ -8,13 +8,6 @@ $clientFilterSchema = is_array($clientFilterSchema ?? null) ? $clientFilterSchem
 $searchConfig = is_array($searchConfig ?? null) ? $searchConfig : null;
 ?>
 <?php
-$breadcrumbs = [
-  ['label' => t('Home'), 'path' => 'admin'],
-  ['label' => t('Search')],
-];
-require templatePath('partials/app-breadcrumb.phtml');
-?>
-<?php
 $listTitle = t('Search results');
 require templatePath('partials/app-list-titlebar.phtml');
 ?>