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.base.css

@@ -602,7 +602,7 @@
   }
 }
 
-[data-theme^="dark"] {
+[data-theme="dark"] {
   color-scheme: dark;
   --app-background-color: rgb(19, 22.5, 30.5);
   --app-sidebar-background-color: var(--app-background-color);
@@ -830,7 +830,7 @@
   --app-icon-invalid: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='24' height='24' viewBox='0 0 24 24' fill='none' stroke='rgb(149.5, 74, 80)' stroke-width='2' stroke-linecap='round' stroke-linejoin='round'%3E%3Ccircle cx='12' cy='12' r='10'%3E%3C/circle%3E%3Cline x1='12' y1='8' x2='12' y2='12'%3E%3C/line%3E%3Cline x1='12' y1='16' x2='12.01' y2='16'%3E%3C/line%3E%3C/svg%3E");
 }
 
-[data-theme^="dark"]
+[data-theme="dark"]
   input:is(
     [type="submit"],
     [type="button"],