[Enhancement] Extend the `words` content with contextual information

### Purpose
Instead of limiting the `words` content to just the word and its meaning, we can enhance it to support learning words in context.

### Proposed New Fields
- `example`: Example sentence (text)
- `word_type`: Part of speech (adjective / noun / verb, etc.)
- `synonym`: Synonymous words (text array)
- `antonym`: Antonymous words (text array)
- `level`: Estimated difficulty level (A1 - C2)

### Why Is This Necessary?
- Users will not only memorize the words but also understand their usage in context.
- This lays the groundwork for future features such as recommendation systems, sorting by difficulty level, and showing related words.
- The word detail view becomes more informative and educational.

Start by locating the existing `words` content and the word detail view. Review how word and meaning data are currently represented, then determine how the proposed example, word type, synonyms, antonyms, and level fields should be supported. Done means the words content and detail view accommodate the proposed contextual information.

Written by the indexing model from the issue text.