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/js/components/app-theme-toggle.js

@@ -8,7 +8,7 @@ const setTheme = (theme) => {
   document.documentElement.dataset.theme = theme;
 };
 
-const isDarkTheme = (theme) => theme && theme.startsWith('dark');
+const isDarkTheme = (theme) => theme === 'dark';
 
 const setIcon = (iconEl, theme) => {
   if (!iconEl) {