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/base/variables.contrast.css

@@ -37,8 +37,8 @@ html[data-contrast="high"][data-theme="light"],
   --app-action-danger-outline-background: rgba(143, 0, 0, 0.2);
 }
 
-html[data-contrast="high"][data-theme^="dark"],
-[data-contrast="high"][data-theme^="dark"] {
+html[data-contrast="high"][data-theme="dark"],
+[data-contrast="high"][data-theme="dark"] {
   --app-background-color: #000;
   --app-sidebar-background-color: var(--app-background-color);
   --app-color: #f8fafc;