feat: module-owned i18n with merge-at-boot translation loading

Each module can now ship its own i18n/ directory with translation files
(default_de.json, default_en.json). At boot time, ModuleI18nLoader
preloads core translations then merges module translations on top via
Reflection into I18n::$strings — the t() helper stays unchanged.

- Add i18nPath property to ModuleManifest
- Add ModuleI18nLoader with Reflection-based preload
- Extract 59 module-exclusive keys from core i18n into module files
  (notifications: 8, bookmarks: 41, addressbook: 10)
- Expand TranslationKeysTest to scan modules/ and validate against
  merged core + module translations
- Add ModuleI18nContractTest for per-module de/en parity and completeness

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/App/Module/ModuleRegistry.php

@@ -148,6 +148,12 @@ final class ModuleRegistry
         // module classes (MintyPHP\Module\<Name>\*) resolve without a custom loader.
         self::registerModuleAutoloading($manifests);
 
+        // Preload core + module translations into I18n before any t() call.
+        $coreI18nDir = rtrim($modulesDir, '/') . '/../i18n';
+        if (is_dir($coreI18nDir)) {
+            ModuleI18nLoader::preload($manifests, $coreI18nDir);
+        }
+
         // Register and merge
         foreach ($manifests as $manifest) {
             $registry->registerModule($manifest);