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>

i18n/default_en.json

@@ -295,7 +295,6 @@
   "Profile image": "Profile image",
   "Profile": "Profile",
   "Profile description": "Profile description",
-  "About": "About",
   "Info": "Info",
   "Search results": "Search results",
   "Result": "Result",
@@ -470,10 +469,6 @@
   "Status & meta": "Status & meta",
   "Audit": "Audit",
   "Overview": "Overview",
-  "Quick actions": "Quick actions",
-  "Send email": "Send email",
-  "Call phone": "Call phone",
-  "Call mobile": "Call mobile",
   "Choose color": "Choose color",
   "Be careful - this resets the tenants color": "Be careful - this resets the tenant color",
   "Code": "Code",
@@ -520,13 +515,11 @@
   "Department can not be created": "Department can not be created",
   "Department can not be updated": "Department can not be updated",
   "Delete this department?": "Delete this department?",
-  "Delete this bookmark?": "Delete this bookmark?",
   "Delete department": "Delete department",
   "This will permanently delete this department.": "This will permanently delete this department.",
   "Assigned departments": "Assigned departments",
   "Select departments": "Select departments",
   "Departments for tenant": "Departments for tenant",
-  "No departments": "No departments",
   "No departments available for this tenant": "No departments available for this tenant",
   "Assign tenants first to select departments": "Assign tenants first to select departments",
   "No tenants available": "No tenants available",
@@ -549,7 +542,6 @@
   "Delete permission": "Delete permission",
   "This will permanently delete this role.": "This will permanently delete this role.",
   "This will permanently delete this permission.": "This will permanently delete this permission.",
-  "This action cannot be undone.": "This action cannot be undone.",
   "Description": "Description",
   "Description cannot be empty": "Description cannot be empty",
   "Status is invalid": "Status is invalid",
@@ -647,7 +639,6 @@
   "SMTP security": "SMTP security",
   "SMTP from address": "SMTP from address",
   "SMTP from name": "SMTP from name",
-  "Address book": "Address book",
   "Mobile": "Mobile",
   "Short dial": "Short dial",
   "setting.smtp_host": "SMTP server host (e.g. smtp.example.com)",
@@ -659,7 +650,6 @@
   "setting.smtp_from_name": "From name for outgoing emails",
   "High contrast": "High contrast",
   "Toggle contrast": "Toggle contrast",
-  "View in address book": "View in address book",
   "Organization & Quality": "Organization & Quality",
   "Email security": "Email security",
   "API": "API",
@@ -813,8 +803,6 @@
   "Please fill required custom field: %s": "Please fill required custom field: %s",
   "Invalid value for %s": "Invalid value for %s",
   "Please select": "Please select",
-  "From": "From",
-  "To": "To",
   "perm.custom_fields.manage": "Manage tenant custom fields",
   "perm.custom_fields.edit_values": "Edit user custom field values",
   "Label": "Label",
@@ -999,7 +987,6 @@
   "Import another file": "Import another file",
   "No row errors found": "No row errors found",
   "Row": "Row",
-  "Code": "Code",
   "Message": "Message",
   "No upload file provided": "No upload file provided",
   "Upload exceeds the maximum size": "Upload exceeds the maximum size",
@@ -1212,7 +1199,6 @@
   "Actor tenant": "Actor tenant",
   "Event details": "Event details",
   "Target type": "Target type",
-  "Request ID": "Request ID",
   "IP hash": "IP hash",
   "User agent hash": "User agent hash",
   "Channel": "Channel",
@@ -1299,45 +1285,5 @@
   "Session expiring": "Session expiring",
   "Your session will expire in {countdown}. Would you like to continue?": "Your session will expire in {countdown}. Would you like to continue?",
   "Extend session": "Extend session",
-  "Log out": "Log out",
-  "Bookmarks": "Bookmarks",
-  "Bookmark this page": "Bookmark this page",
-  "Save bookmark": "Save bookmark",
-  "Bookmark name": "Bookmark name",
-  "Delete bookmark": "Delete 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"
+  "Log out": "Log out"
 }