feat(VocabPracticeDialog, localization): add hard vocabulary marking feature

- Introduced functionality to mark vocabulary items as difficult, allowing users to track and manage challenging words during practice sessions.
- Added buttons for marking and unmarking current vocabulary as hard, enhancing user engagement and learning effectiveness.
- Updated the UI to display the count of hard-marked vocabulary items, providing users with better insights into their learning progress.
- Enhanced localization files for German and English to support new vocabulary marking features, ensuring a consistent user experience across languages.

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

@@ -461,6 +461,9 @@
                 "remaining": "Noch offen",
                 "success": "Erfolg",
                 "fail": "Misserfolg",
+                "hardCount": "Schwer markiert",
+                "markHard": "Als schwer markieren",
+                "unmarkHard": "Aus schwer entfernen",
                 "srsRateTitle": "Wie sicher war das?",
                 "srsAgain": "Nochmal",
                 "srsAgainHint": "sehr bald wiederholen",

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

@@ -461,6 +461,9 @@
                 "remaining": "Remaining",
                 "success": "Success",
                 "fail": "Fail",
+                "hardCount": "Marked difficult",
+                "markHard": "Mark as difficult",
+                "unmarkHard": "Remove difficult mark",
                 "srsRateTitle": "How solid did it feel?",
                 "srsAgain": "Again",
                 "srsAgainHint": "repeat very soon",