feat: FLUX Klein 4B editor, path traversal hardening, and UI polish

Add FLUX Klein 4B for AI object removal and text-guided video editing
via auto_mask skill. Per-frame processing with temporal smoothing,
subprocess isolation, and automatic model download from HuggingFace
mirror. Security: validate_video_path on all entry points including
_probe_duration. Minor UI polish (aria-labels, composer optimization).

Author: AEmotionStudio