feat(l10n): add hint text for existing comments

- Add new localization entries for both Arabic and English
- Implement hint text to inform users they have already commented on a headline
- Update existing comment input field descriptions in app_ar.arb and app_en.arb

Author: fulleni

lib/l10n/arb/app_ar.arb

@@ -2042,5 +2042,9 @@
     "commentInputDisabledHint": "سجل الدخول للانضمام إلى المحادثة",
     "@commentInputDisabledHint": {
         "description": "Hint text for the comment input field when the user is a guest."
+    },
+    "commentInputExistingHint": "لقد علقت بالفعل على هذا العنوان.",
+    "@commentInputExistingHint": {
+        "description": "Hint text for the comment input field when the user has already posted a comment."
     }
 }

lib/l10n/arb/app_en.arb

@@ -2042,5 +2042,9 @@
     "commentInputDisabledHint": "Sign in to join the conversation",
     "@commentInputDisabledHint": {
         "description": "Hint text for the comment input field when the user is a guest."
+    },
+    "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."
     }
 }