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/es/socialnetwork.json

@@ -448,6 +448,8 @@
                 "corePatternsHint": "Primero la lengua meta; debajo, el significado en tu idioma.",
                 "vocabPrepTitle": "Preparación antes del entrenador de vocabulario",
                 "vocabPrepStep1": "Lee una vez los patrones clave y la lista de palabras (idioma nativo ↔ lengua meta).",
+                "vocabPrepProgress": "Pasada {pass}: término {current} de {total}",
+                "vocabPrepNextItem": "Siguiente término",
                 "vocabPrepConfirm1": "Primera lectura hecha",
                 "vocabPrepStep2": "Repasa los mismos elementos otra vez (repaso activo, aún sin practicar).",
                 "vocabPrepConfirm2": "Segunda lectura hecha",