Add native macOS SwiftUI GUI

[Anasslaktiri]

Summary

This PR adds a native macOS SwiftUI application (macos-gui/) that wraps the untrunc CLI with a graphical interface — no terminal required.

• Drag & drop reference file and corrupt video file onto labelled drop zones
• All key CLI flags exposed as checkboxes: -s (step unknown), -sv (stretch video), -k (keep unknown), -dw (analysis only), -ms (make streamable), plus a log-level picker
• Live log output streamed in real time from the untrunc subprocess
• Show in Finder button appears automatically when repair completes
• Fully self-contained DMG installer: bundle_libs.sh copies all FFmpeg dylibs into Contents/Frameworks/ and rewrites install names to @loader_path-relative paths — the app runs on any macOS 13+ Apple Silicon machine with no Homebrew or FFmpeg required

How to build

# Requires: Xcode CLT + brew install ffmpeg
cd macos-gui
bash build.sh        # → Untrunc.app

# To produce a distributable DMG (requires: brew install create-dmg):
bash package_dmg.sh  # → Untrunc-1.0.dmg

Test plan

• bash build.sh compiles cleanly and Untrunc.app launches
• Drop zones accept .mp4, .mov, .m4v, .3gp files
• Repair runs and log output streams correctly
• Output _fixed file is produced and "Show in Finder" reveals it
• bash package_dmg.sh produces a working DMG with correct window layout
• App runs on a machine without Homebrew/FFmpeg (dylibs self-contained)

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