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>

modules/notifications/i18n/default_de.json

@@ -0,0 +1,10 @@
+{
+  "Notifications": "Benachrichtigungen",
+  "No notifications": "Keine Benachrichtigungen",
+  "Mark all as read": "Alle als gelesen markieren",
+  "Mark as read": "Als gelesen markieren",
+  "Dismiss": "Verwerfen",
+  "Action failed": "Aktion fehlgeschlagen",
+  "New user: %s": "Neuer Benutzer: %s",
+  "User deleted: %s": "Benutzer gelöscht: %s"
+}