Refactor vocabulary search functionality in VocabService and update UI components

- Modified the searchVocabs method in VocabService to consolidate search parameters into a single query term for improved flexibility.
- Updated VocabSearchDialog to replace separate input fields for mother tongue and learning language with a unified search term input.
- Adjusted button logic to enable search only when a term is provided, enhancing user experience.
- Added new translations for the search term in both German and English locales to support the updated UI.

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

@@ -312,6 +312,7 @@
             "search": {
                 "open": "Suche",
                 "title": "Vokabeln suchen",
+                "term": "Suchbegriff",
                 "motherTongue": "Muttersprache",
                 "learningLanguage": "Lernsprache",
                 "lesson": "Lektion",

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

@@ -312,6 +312,7 @@
             "search": {
                 "open": "Search",
                 "title": "Search vocabulary",
+                "term": "Search term",
                 "motherTongue": "Mother tongue",
                 "learningLanguage": "Learning language",
                 "lesson": "Lesson",