refactor(theme): drop dark-green and collapse theme registry to light/dark

Remove the extensible theme catalog (config/themes.php + ThemeConfigGateway)
in favor of a two-entry const on SettingsAppGateway. appThemes() now returns
the list directly — no container lookup, no file include. Drop the
dark-green theme assets, narrow [data-theme^="dark"] selectors to
[data-theme="dark"], and tighten isDarkTheme() to an exact match. Ship an
idempotent migration that corrects any leftover 'dark-green' rows on users
and tenants to safe defaults (light / NULL).

Tenant scoping (default_theme, allow_user_theme) and per-user override stay
intact; only the catalog extensibility and the third theme are removed.

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

core/Service/Settings/SettingsAppGateway.php

@@ -13,15 +13,18 @@ use MintyPHP\I18n;
  *
  * The theme CATALOG utilities (listThemes, isAllowedTheme, normalizeTheme,
  * resolveDefaultTheme) stay because consumers across Tenant / Auth / User
- * flows need to validate a theme value and resolve a safe default. There is
- * no global setting lookup in these helpers anymore — the default is a
- * hardcoded 'light' falling back to the first available catalog entry.
+ * flows need to validate a theme value and resolve a safe default. The
+ * catalog is a fixed two-entry list; 'light' is the hardcoded default.
  */
 class SettingsAppGateway
 {
+    private const THEMES = [
+        'light' => 'Light',
+        'dark' => 'Dark',
+    ];
+
     public function __construct(
         private readonly SettingsMetadataGateway $settingsMetadataGateway,
-        private readonly ThemeConfigGateway $themeConfigService
     ) {
     }
 
@@ -82,14 +85,11 @@ class SettingsAppGateway
     }
 
     /**
-     * Catalog of available themes — purely code-level (config/themes.php),
-     * not a global setting.
-     *
      * @return array<string, string>
      */
     public function listThemes(): array
     {
-        return $this->themeConfigService->all();
+        return self::THEMES;
     }
 
     public function isAllowedTheme(?string $theme): bool