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>

tests/Support/ThemeResolutionTest.php

@@ -4,7 +4,6 @@ namespace MintyPHP\Tests\Support;
 
 use MintyPHP\App\AppContainer;
 use MintyPHP\Service\Settings\SettingCacheService;
-use MintyPHP\Service\Settings\ThemeConfigGateway;
 use PHPUnit\Framework\TestCase;
 
 /**
@@ -27,10 +26,6 @@ class ThemeResolutionTest extends TestCase
 
         $this->container = new AppContainer();
 
-        $themeConfig = $this->createMock(ThemeConfigGateway::class);
-        $themeConfig->method('all')->willReturn(['light' => 'Light', 'dark' => 'Dark']);
-        $this->container->set(ThemeConfigGateway::class, fn () => $themeConfig);
-
         $this->setAppSettings([]);
 
         $this->pushAppContainer($this->container);