Add filters for story generation

[Yegor-Budnikov]

Add filters:

• by style
• by complexity
• by lengths
• types of characters (animals, people)
• types of plot (structure)

[Yegor-Budnikov]

The first implementation will be simple -- standard drop-down menus for each filter.

Frontend Changes:

1. New dropdown filters (style, complexity, length, character types, plot structure) are added.
2. The selected filters are sent along with the prompt to the backend.

Backend Changes:

1. The server now processes the filters and adds them to the prompt that is sent to the OpenAI API.
2. The filters modify how the story is generated (e.g., length, style, etc.).