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>

web/css/layout/app-topbar.css

@@ -122,6 +122,10 @@
   }
 
   @media (max-width: 576px) {
+    .app-topbar-left .app-breadcrumb {
+      display: none;
+    }
+
     .app-topbar-search-trigger-text,
     .app-topbar-search-trigger-shortcut {
       display: none;
@@ -160,9 +164,7 @@
       color var(--app-transition);
   }
 
-  .app-topbar-left > li > a,
   .app-topbar-right > li > a,
-  .app-topbar-left button,
   .app-topbar-right button {
     display: flex;
     align-items: center;
@@ -181,39 +183,26 @@
       color var(--app-transition);
   }
 
-  .app-topbar-left > li > a:hover,
   .app-topbar-right > li > a:hover,
-  .app-topbar-left button:hover,
   .app-topbar-right button:hover,
   .app-header details.dropdown > summary:not([role]):hover {
     background: color-mix(in srgb, var(--app-contrast) 8%, transparent);
     color: var(--app-color);
   }
 
-  .app-topbar-left > li > a:active,
   .app-topbar-right > li > a:active,
-  .app-topbar-left button:active,
   .app-topbar-right button:active,
   .app-header details.dropdown > summary:not([role]):active {
     background: color-mix(in srgb, var(--app-contrast) 12%, transparent);
   }
 
-  .app-topbar-left > li > a:focus-visible,
   .app-topbar-right > li > a:focus-visible,
-  .app-topbar-left button:focus-visible,
   .app-topbar-right button:focus-visible,
   .app-header details.dropdown > summary:not([role]):focus-visible {
     outline: 2px solid var(--app-primary);
     outline-offset: -2px;
   }
 
-  .app-header #global-back.is-disabled,
-  .app-header #global-forward.is-disabled {
-    opacity: 0.4;
-    cursor: not-allowed;
-    pointer-events: none;
-  }
-
   .app-topbar-tenant-chip {
     display: inline-flex;
     align-items: center;