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>

config/assets.php

@@ -6,7 +6,6 @@ return [
             'base' => [
                 'css/app-layers.css',
                 'css/base/variables.base.css',
-                'css/base/variables.theme-dark-green.css',
                 'css/base/variables.contrast.css',
             ],
             'shared' => [

config/themes.php

@@ -1,7 +0,0 @@
-<?php
-
-return [
-    'light' => 'Light',
-    'dark' => 'Dark',
-    'dark-green' => 'Dark green',
-];