feat(vocab): add grammar explanations to VocabLessonView and enhance localization

- Introduced new sections for grammar explanations in VocabLessonView, providing users with contextual insights during vocabulary lessons.
- Added localization keys in German, English, and Spanish for the new grammar explanation titles and introductions, improving user guidance across languages.
- Updated styles for the grammar explanation section to enhance visual clarity and user experience.

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

@@ -461,6 +461,8 @@
                 "learningGoals": "Learning Goals",
                 "corePatterns": "Core Patterns",
                 "corePatternsHint": "Read the target language first, then the meaning below — you learn each pattern both ways.",
+                "learningGrammarTitle": "Briefly frame the grammar",
+                "learningGrammarIntro": "Read these 1-2 hints after the key terms. Then you go into the trainer with better orientation.",
                 "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}",