fix(general): route supported-languages field through LocaleSyncService, guard unsupported default locale

General's free-text 'Supported Languages' field overwrote
tenant/localization.supportedLocales directly, skipping LocaleSyncService's
propagation to per-locale nav/static-page translation entries - the exact
sync Languages' add/remove buttons already go through correctly. Now diffs
against the current list and routes each added/removed locale through
facade.addLocale()/removeLocale().

Also: 'Default Language' was a free-text input with no guard against typing
a locale that isn't in the supported list - every label[defaultLocale]
lookup across nav/static-page content would then silently return undefined.
Added a validator rule (default-locale-not-supported) wired to the existing
fieldError() display, consistent with every other field-level check.

Verified live via window.ng.getComponent(): typing an unsupported code shows
the new inline error; adding 'de' via this field seeded an empty 'de'
translation entry on an existing static page, matching what Languages'
add-locale button already produces.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/i18n/translations.ts

@@ -536,6 +536,7 @@ export interface Translations {
     featureFlags: string;
     validationMissingLogo: string;
     validationNoLanguages: string;
+    validationDefaultLocaleNotSupported: string;
     validationInvalidUrl: string;
     validationDuplicateSlugs: string;
     validationEmptyHomepage: string;