refactor(settings): collapse get_all to aksrt_settings and drop migration UI

Make aksrt_settings the sole theme settings store for get_all(), remove
aksrt_seo_settings/aksrt_basic_settings merge paths, point global.logo
schema at aksrt_settings, and delete the retired settings migration page,
route, REST endpoint, and related SPA surface.

inc/classes/class-aksrt-settings.php

@@ -66,7 +66,7 @@ class AKSRT_Settings {
      */
     public static function on_option_updated(string $option_name): void {
         $tracked = array_merge(
-            ['aksrt_settings', 'aksrt_basic_settings'],
+            ['aksrt_settings'],
             self::STANDALONE_OPTION_KEYS
         );
 
@@ -95,19 +95,14 @@ class AKSRT_Settings {
             return self::$cached_all;
         }
 
+        // Canonical theme settings live only in aksrt_settings.
+        // Operational configs (captcha/moderation/oauth/footer/etc.) stay standalone.
         $settings = get_option('aksrt_settings', []);
-        $seo      = get_option('aksrt_seo_settings', []);
-        $basic    = get_option('aksrt_basic_settings', []);
-
-        $merged = array_merge(
-            is_array($settings) ? $settings : [],
-            is_array($seo) ? $seo : [],
-            is_array($basic) ? $basic : []
-        );
-
-        if (!is_array($merged)) $merged = [];
-        self::$cached_all = $merged;
-        return $merged;
+        if (!is_array($settings)) {
+            $settings = [];
+        }
+        self::$cached_all = $settings;
+        return $settings;
     }
 
     /**
@@ -126,7 +121,6 @@ class AKSRT_Settings {
      * 用于导入导出完整性检查。
      */
     public const STANDALONE_OPTION_KEYS = [
-        'aksrt_seo_settings',
         'aksrt_mail_smtp',
         'aksrt_oauth',
         'aksrt_captcha_config',
@@ -267,7 +261,7 @@ class AKSRT_Settings {
         }
     }
 
-    /** 启动时对齐原生 site_icon，兼容历史已保存的 SEO favicon。 */
+    /** 启动时对齐原生 site_icon，同步自定义 site_favicon 到原生 site_icon。 */
     public static function maybe_sync_native_site_icon(): void {
         $custom = self::custom_favicon_url();
         if ($custom === '') {