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/features/project-editor/sections/general-section.component.ts

@@ -61,12 +61,25 @@ export class ProjectEditorGeneralSectionComponent {
     }));
   }
 
+  /**
+   * Routes through facade.addLocale()/removeLocale() (LocaleSyncService)
+   * instead of overwriting supportedLocales directly - a direct overwrite
+   * skipped seeding/cleaning up per-locale nav and static-page translation
+   * entries, leaving them out of sync with what this field claims is
+   * supported (and Languages' add/remove buttons already do it correctly).
+   */
   updateSupportedLanguages(value: string): void {
-    const locales = value.split(',').map(item => item.trim()).filter(Boolean);
-    this.facade.updateBootstrap(current => ({
-      ...current,
-      tenant: { ...current.tenant, supportedLocales: locales },
-      localization: { ...current.localization, supportedLocales: locales }
-    }));
+    const next = new Set(value.split(',').map(item => item.trim().toLowerCase()).filter(Boolean));
+    const current = this.bootstrap()?.localization.supportedLocales ?? [];
+    for (const locale of current) {
+      if (!next.has(locale)) {
+        this.facade.removeLocale(locale);
+      }
+    }
+    for (const locale of next) {
+      if (!current.includes(locale)) {
+        this.facade.addLocale(locale);
+      }
+    }
   }
 }