feat(l10n): add commenterName string for missing user objects

- Add new localization string for displaying commenter name when full user object is not available
- Implement in both Arabic (app_ar.arb) and English (app_en.arb) localization files
- Include placeholder for partial user ID in the string

Author: fulleni

lib/l10n/arb/app_ar.arb

@@ -2046,5 +2046,15 @@
     "commentInputExistingHint": "لقد علقت بالفعل على هذا العنوان.",
     "@commentInputExistingHint": {
         "description": "Hint text for the comment input field when the user has already posted a comment."
+    },
+    "commenterName": "مستخدم {id}",
+    "@commenterName": {
+        "description": "Display name for a commenter when their full user object is not available, showing a partial ID.",
+        "placeholders": {
+            "id": {
+                "type": "String",
+                "example": "1a2b"
+            }
+        }
     }
 }

lib/l10n/arb/app_en.arb

@@ -2046,5 +2046,15 @@
     "commentInputExistingHint": "You have already commented on this headline.",
     "@commentInputExistingHint": {
         "description": "Hint text for the comment input field when the user has already posted a comment."
+    },
+    "commenterName": "User {id}",
+    "@commenterName": {
+        "description": "Display name for a commenter when their full user object is not available, showing a partial ID.",
+        "placeholders": {
+            "id": {
+                "type": "String",
+                "example": "1a2b"
+            }
+        }
     }
 }