Add Linux AppImage support

[cypher-phunk]

This PR ports QualityScaler to Linux, distributed as a self-contained AppImage that requires no installation beyond an NVIDIA driver.

Changes:

• QualityScaler.py — Linux compatibility fixes: iconphoto() instead of iconbitmap() (Linux only accepts .xbm), native file dialogs via zenity/kdialog with tkinter fallback, corrected subprocess calls (shell=False bool — the original shell="False" string is truthy and caused ffmpeg to receive no arguments)
• requirements.txt — replaces onnxruntime-directml with onnxruntime-gpu 1.18.1 for NVIDIA CUDA acceleration. Pinned to 1.18.1 because 1.19+ uses cuDNN 9 which dropped Pascal GPU support (sm_61, GTX 10xx)
• build-appimage.sh — builds a self-contained AppImage bundling Python, CUDA libraries, static ffmpeg/exiftool, and AI models
• QualityScaler.spec — PyInstaller spec updated for Linux, includes PIL.ImageTk hidden imports (system Pillow on Linux is headless)
• .github/workflows/release.yml — builds and publishes the AppImage on tag push
• README.md — added Linux prerequisites and getting started section