app.py relied on parsing stdout for the save path printed by the
pipeline. After PR #21 replaced that print with logger.info(), the
stdout parsing returned None, causing "Aucune image générée détectée".
Now check the known save path directly (APP_DIR / save_path) and
only fall back to stdout parsing if the file isn't found.
_extract_image_from_stdout() failed to find the generated image because
the pipeline prints "Card generated and saved to: generated_card.png"
but the function only tried the full line as a path. Now also tries the
part after the last colon.
Fix two critical bugs that made the Streamlit app completely non-functional:
1. Split subprocess command into properly separated list elements and use
sys.executable instead of hardcoded "python" (#1)
2. Unpack all 3 return values from run_prompt_pipeline() (#2)
Closes#1, closes#2
