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>

core/Support/helpers/admin_settings.php

@@ -0,0 +1,47 @@
+<?php
+
+/**
+ * Merge a single-section POST body with current DB values so that
+ * AdminSettingsService::updateFromAdmin() only updates the fields owned
+ * by that section.
+ *
+ * Why this exists:
+ * updateFromAdmin() reads all known keys from the POST array and defaults
+ * missing keys to 0/empty/false (see sanitize* methods). Subpages that post
+ * only their own fields would otherwise wipe unrelated sections. This helper
+ * overlays the section's POST fields onto the current DB values formatted as
+ * POST scalars, preserving checkbox semantics (omitted key = unchecked).
+ *
+ * @param array<string, mixed> $currentValues  'values' map from buildPageData()
+ * @param array<string, mixed> $post           user POST body (bodyAll())
+ * @param list<string>         $sectionKeys    keys owned by this section
+ * @return array<string, mixed>
+ */
+function settingsSectionMergePost(array $currentValues, array $post, array $sectionKeys): array
+{
+    $seed = [];
+    foreach ($currentValues as $key => $value) {
+        if (is_bool($value)) {
+            if ($value) {
+                $seed[$key] = '1';
+            }
+            continue;
+        }
+        if (is_array($value)) {
+            $seed[$key] = $value;
+            continue;
+        }
+        if ($value !== null) {
+            $seed[$key] = (string) $value;
+        }
+    }
+
+    foreach ($sectionKeys as $key) {
+        unset($seed[$key]);
+        if (array_key_exists($key, $post)) {
+            $seed[$key] = $post[$key];
+        }
+    }
+
+    return $seed;
+}