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>

web/css/vendor-overrides/gridjs.css

@@ -691,11 +691,6 @@
     filter: invert(1);
   }
 
-  html[data-theme="dark-green"] button.gridjs-sort,
-  html[data-theme="dark-green"] button.gridjs-sort-neutral {
-    filter: invert(1);
-  }
-
   button.gridjs-sort {
     background-color: transparent;
     background-position-x: center;