[Bug]: Invalid default model strings in keys.py (gpt-5.4-mini, mixtral-8x7b-32768)

Problem

Two default model strings in manga_translator/translators/keys.py are invalid and cause 404 model_not_found for users who don't set the env vars.

File cite

• manga_translator/translators/keys.py:15 — OPENAI_MODEL = os.getenv('OPENAI_MODEL', 'gpt-5.4-mini')
• manga_translator/translators/keys.py:18 — GROQ_MODEL = os.getenv('GROQ_MODEL', 'mixtral-8x7b-32768')

gpt-5.4-mini is not and never has been a valid OpenAI model id. mixtral-8x7b-32768 was deprecated and removed from Groq's API in 2024.

Steps

1. pip install -r requirements.txt
2. Set only OPENAI_API_KEY (or GROQ_API_KEY); leave OPENAI_MODEL / GROQ_MODEL unset
3. Run any translation with --translator chatgpt (or groq)

Expected

Successful API call using a working default model.

Actual

openai.NotFoundError: Error code: 404 - {'error': {'message': 'The model \gpt-5.4-mini` does not exist ...'}}` (analogous for Groq).

Environment

• Python >=3.10, <3.12 (pyproject.toml)
• openai==1.63.0, groq (unpinned) per requirements.txt

Suggested fix: default to gpt-4o-mini and llama-3.1-8b-instant respectively.

Thanks for maintaining zyddnys/manga-image-translator!