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/ModuleManifest.php

@@ -83,6 +83,8 @@ final class ModuleManifest
 
     public readonly ?string $migrationsPath;
 
+    public readonly ?string $i18nPath;
+
     public readonly string $basePath;
 
     /**
@@ -128,6 +130,11 @@ final class ModuleManifest
         $this->migrationsPath = is_string($migrationsPath) && trim($migrationsPath) !== ''
             ? rtrim($basePath, '/') . '/' . ltrim(trim($migrationsPath), '/')
             : null;
+
+        $i18nPath = $raw['i18n_path'] ?? null;
+        $this->i18nPath = is_string($i18nPath) && trim($i18nPath) !== ''
+            ? rtrim($basePath, '/') . '/' . ltrim(trim($i18nPath), '/')
+            : null;
     }
 
     /**