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/bookmarks/i18n/default_de.json

@@ -0,0 +1,43 @@
+{
+  "Bookmarks": "Lesezeichen",
+  "Bookmark this page": "Seite als Lesezeichen speichern",
+  "Save bookmark": "Lesezeichen speichern",
+  "Bookmark name": "Lesezeichen-Name",
+  "Delete bookmark": "Lesezeichen löschen",
+  "Delete this bookmark?": "Dieses Lesezeichen wirklich löschen?",
+  "No bookmarks yet": "Noch keine Lesezeichen",
+  "New group": "Neue Gruppe",
+  "No group": "Keine Gruppe",
+  "Assign to group": "Gruppe zuweisen",
+  "Group name": "Gruppenname",
+  "Delete group": "Gruppe löschen",
+  "Group": "Gruppe",
+  "Icon": "Symbol",
+  "Maximum bookmarks reached": "Maximale Anzahl an Lesezeichen erreicht",
+  "Edit bookmark": "Lesezeichen bearbeiten",
+  "Save changes": "Änderungen speichern",
+  "Bookmark saved": "Lesezeichen gespeichert",
+  "Bookmark updated": "Lesezeichen aktualisiert",
+  "Bookmark deleted": "Lesezeichen gelöscht",
+  "Group created": "Gruppe erstellt",
+  "Group deleted": "Gruppe gelöscht",
+  "Group delete failed": "Gruppe konnte nicht gelöscht werden",
+  "Delete group and keep bookmarks?": "Gruppe löschen und enthaltene Lesezeichen behalten?",
+  "Bookmark action failed": "Lesezeichen-Aktion fehlgeschlagen",
+  "Group action failed": "Gruppen-Aktion fehlgeschlagen",
+  "Reorder saved": "Sortierung gespeichert",
+  "Reorder failed": "Sortierung fehlgeschlagen",
+  "Move group up": "Gruppe nach oben",
+  "Move group down": "Gruppe nach unten",
+  "Move bookmark up": "Lesezeichen nach oben",
+  "Move bookmark down": "Lesezeichen nach unten",
+  "Group actions": "Gruppenaktionen",
+  "Bookmark actions": "Lesezeichenaktionen",
+  "Bookmark name is required": "Lesezeichen-Name ist erforderlich",
+  "Bookmark URL is invalid": "Lesezeichen-URL ist ungültig",
+  "Group not found": "Gruppe nicht gefunden",
+  "Edit group": "Gruppe bearbeiten",
+  "Group updated": "Gruppe aktualisiert",
+  "Group name is required": "Gruppenname ist erforderlich",
+  "Maximum groups reached": "Maximale Anzahl an Gruppen erreicht"
+}

modules/bookmarks/i18n/default_en.json

@@ -0,0 +1,43 @@
+{
+  "Bookmarks": "Bookmarks",
+  "Bookmark this page": "Bookmark this page",
+  "Save bookmark": "Save bookmark",
+  "Bookmark name": "Bookmark name",
+  "Delete bookmark": "Delete bookmark",
+  "Delete this bookmark?": "Delete this bookmark?",
+  "No bookmarks yet": "No bookmarks yet",
+  "New group": "New group",
+  "No group": "No group",
+  "Assign to group": "Assign to group",
+  "Group name": "Group name",
+  "Delete group": "Delete group",
+  "Group": "Group",
+  "Icon": "Icon",
+  "Maximum bookmarks reached": "Maximum bookmarks reached",
+  "Edit bookmark": "Edit bookmark",
+  "Save changes": "Save changes",
+  "Bookmark saved": "Bookmark saved",
+  "Bookmark updated": "Bookmark updated",
+  "Bookmark deleted": "Bookmark deleted",
+  "Group created": "Group created",
+  "Group deleted": "Group deleted",
+  "Group delete failed": "Group delete failed",
+  "Delete group and keep bookmarks?": "Delete group and keep bookmarks?",
+  "Bookmark action failed": "Bookmark action failed",
+  "Group action failed": "Group action failed",
+  "Reorder saved": "Reorder saved",
+  "Reorder failed": "Reorder failed",
+  "Move group up": "Move group up",
+  "Move group down": "Move group down",
+  "Move bookmark up": "Move bookmark up",
+  "Move bookmark down": "Move bookmark down",
+  "Group actions": "Group actions",
+  "Bookmark actions": "Bookmark actions",
+  "Bookmark name is required": "Bookmark name is required",
+  "Bookmark URL is invalid": "Bookmark URL is invalid",
+  "Group not found": "Group not found",
+  "Edit group": "Edit group",
+  "Group updated": "Group updated",
+  "Group name is required": "Group name is required",
+  "Maximum groups reached": "Maximum groups reached"
+}

modules/bookmarks/module.php

@@ -130,4 +130,5 @@ return [
 
     'permissions' => [],
     'migrations_path' => null,
+    'i18n_path' => 'i18n',
 ];