feat(localization): add difficult vocabulary marking features in multiple languages

- Introduced new localization strings for marking vocabulary as difficult, including options to save and remove difficult marks.
- Enhanced user experience by providing feedback on the status of difficult vocabulary in Cebuano, Spanish, and French.
- Updated existing localization files to ensure consistency across languages, supporting the recent vocabulary management features.

frontend/src/i18n/locales/ceb/socialnetwork.json

@@ -503,6 +503,13 @@
                 "trainerProgressNewContent": "Bag-ong sulod: {current}/{target}",
                 "trainerProgressReview": "Balik-balik: {count}",
                 "trainerProgressMixShare": "Nasagol nga bahin: {percent}%",
+                "markVocabHard": "Mark as difficult",
+                "markVocabHardSaved": "Vocabulary marked as difficult.",
+                "unmarkVocabHard": "Remove from difficult list",
+                "unmarkVocabHardSaved": "Vocabulary removed from difficult list.",
+                "hardVocabModeActive": "Intensive block: difficult vocabulary",
+                "hardVocabRemaining": "Remaining until stable: {count}",
+                "startHardVocabTrainer": "Train difficult vocabulary ({count})",
                 "unknownExerciseTypeNotice": "Kini nga matang sa ehersisyo wala pa ipakita nga interaktibo sa kasamtangang view.",
                 "unknownExerciseTypeLabel": "Matang: {type}",
                 "lessonReviewHeadlineDone": "Nakaabot na kini nga leksiyon sa libre nga pagpalalom.",
@@ -806,6 +813,9 @@
                 "remaining": "Nahibilin",
                 "success": "Malampuson",
                 "fail": "Fail",
+                "hardCount": "Marked difficult",
+                "markHard": "Mark as difficult",
+                "unmarkHard": "Remove difficult mark",
                 "srsRateTitle": "Unsa ka lig-on sa imong pagbati?",
                 "srsAgain": "Usab",
                 "srsAgainHint": "balikon dayon",