feat(vocabService, VocabPracticeDialog, VocabLessonView): enhance vocabulary handling and exposure tracking

- Updated vocabService to merge extracted vocabularies and improve handling of learning and reference pairs.
- Introduced normalization and exposure tracking in VocabPracticeDialog to ensure diverse and underexposed vocabulary practice.
- Enhanced VocabLessonView with methods to identify underexposed vocabularies and adjust selection logic for improved learning outcomes.
- Implemented new constants for minimum exposure requirements to optimize vocabulary training sessions.

backend/scripts/update-bisaya-didactics.js

@@ -321,17 +321,17 @@ export const LESSON_DIDACTICS = {
       { target: 'Duha', gloss: 'zwei' },
       { target: 'Tulo', gloss: 'drei' },
       { target: 'Upat', gloss: 'vier' },
-      { target: 'Lima', gloss: 'fuenf' },
+      { target: 'Lima', gloss: 'fünf' },
       { target: 'Unom', gloss: 'sechs' },
       { target: 'Pito', gloss: 'sieben' },
       { target: 'Walo', gloss: 'acht' },
       { target: 'Siyam', gloss: 'neun' },
       { target: 'Napulo', gloss: 'zehn' },
       { target: 'Napulo ug usa', gloss: 'elf' },
-      { target: 'Napulo ug duha', gloss: 'zwoelf' },
+      { target: 'Napulo ug duha', gloss: 'zwölf' },
       { target: 'Napulo ug tulo', gloss: 'dreizehn' },
       { target: 'Napulo ug upat', gloss: 'vierzehn' },
-      { target: 'Napulo ug lima', gloss: 'fuenfzehn' },
+      { target: 'Napulo ug lima', gloss: 'fünfzehn' },
       { target: 'Napulo ug unom', gloss: 'sechzehn' },
       { target: 'Napulo ug pito', gloss: 'siebzehn' },
       { target: 'Napulo ug walo', gloss: 'achtzehn' },