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/Support/helpers/branding.php

@@ -7,18 +7,16 @@
  */
 
 /**
- * Load configured themes with a safe fallback list.
+ * Available themes. Fixed two-entry catalog — light and dark.
+ *
+ * @return array<string, string>
  */
 function appThemes(): array
 {
-    if (!class_exists('MintyPHP\\Service\\Settings\\ThemeConfigGateway')) {
-        return [
-            'light' => 'Light',
-            'dark' => 'Dark',
-        ];
-    }
-
-    return app(\MintyPHP\Service\Settings\ThemeConfigGateway::class)->all();
+    return [
+        'light' => 'Light',
+        'dark' => 'Dark',
+    ];
 }
 
 /**