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>

db/updates/2026-04-24-theme-simplification.sql

@@ -0,0 +1,7 @@
+-- Theme simplification: collapse catalog to light + dark.
+-- Migrates any leftover 'dark-green' values back to safe defaults.
+-- Idempotent — re-runs are a no-op because the WHERE clauses filter
+-- on the exact legacy value that the preceding run already replaced.
+
+UPDATE users SET theme = 'light' WHERE theme = 'dark-green';
+UPDATE tenants SET default_theme = NULL WHERE default_theme = 'dark-green';