fix: skip invalid images during SAM preload - #264

Fixes #69.

A failed image decode returned a null QImage from the auto-labeling helper. The SAM background preloader then passed it to qimage2ndarray, which raised the exact reported ValueError: qimageview got invalid QImage.

This change returns None after empty or invalid image data so the existing preload guard skips the file. It also adds regression coverage for corrupt and valid image files.

Validation:

• Linux: full suite, 137 passed / 13 optional model tests skipped
• Linux: manual corrupt-image SAM preload; encoder and cache remained untouched
• Apple Silicon macOS: focused tests and manual corrupt-image SAM preload passed
• Windows: focused corrupt/valid image tests passed
• ruff check and ruff format --check passed