feat(vocab-prep): implement enhanced vocabulary preparation steps in VocabLessonView

- Introduced a structured vocabulary preparation process with two review stages before engaging with the vocabulary trainer.
- Added localization support for new vocabulary preparation messages in German, English, and Spanish, improving accessibility for users.
- Updated the VocabLessonView component to display current progress and next steps during vocabulary preparation, enhancing user guidance.
- Refactored related logic to manage preparation stages and item navigation effectively.

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

@@ -450,6 +450,8 @@
                 "corePatternsHint": "Read the target language first, then the meaning below — you learn each pattern both ways.",
                 "vocabPrepTitle": "Preparation before the vocabulary trainer",
                 "vocabPrepStep1": "Read through core patterns and the word list (native language ↔ target language) once.",
+                "vocabPrepProgress": "Pass {pass}: item {current} of {total}",
+                "vocabPrepNextItem": "Next item",
                 "vocabPrepConfirm1": "First pass done",
                 "vocabPrepStep2": "Go through the same items again (active review, not testing yet).",
                 "vocabPrepConfirm2": "Second pass done",