refactor(settings): split admin/settings into hub + per-section subpages

Splits the 755-line monolithic settings page into a tile-based landing hub
and six focused subpages (general, security, email, api, sso, branding),
each with its own form, CSRF scope and POST handler. Each subpage offers
Save / Save & close buttons plus a Cancel/back link to the hub.

Backend (AdminSettingsService, gateways, policies, DB schema) unchanged.
A new settingsSectionMergePost() helper overlays section POSTs onto the
current DB values so partial saves don't wipe unrelated fields (the
service defaults missing keys to 0/empty).

Sub-action files (logo/favicon/tokens/lifecycle) redirect to the matching
subpage, and architecture contracts now check the subpage files instead
of the removed monolithic index.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/Architecture/FrontendRuntimeHostContractTest.php

@@ -45,7 +45,6 @@ class FrontendRuntimeHostContractTest extends TestCase
             'pages/admin/departments/_form.phtml',
             'pages/admin/permissions/_form.phtml',
             'pages/admin/roles/_form.phtml',
-            'pages/admin/settings/index(default).phtml',
             'pages/admin/stats/index(default).phtml',
             'pages/admin/tenants/_form.phtml',
             'pages/admin/users/_form.phtml',