Add native settings bridge, CLI auto-detection, error handlers and CLI-aware buttons; improve macOS menu & layout fixes

• Add native settings integration:

  • Introduce SettingsService / Settings bridge on macOS for get/set settings and showSettings.
  • Add macOS native window code (SettingsWindow.swift/MainFlutterWindow changes) and AppDelegate wiring to insert a Settings menu item and a settings method channel.
  • Provide methods to show a native Settings window and handle setting change notifications; add logging for settings flows.

• CLI improvements:

  • Convert initCli to async and call it at startup.
  • Make Cli.init asynchronous and add auto-detection of common CLI tools (ffmpeg, ffprobe, gallery-dl, gifski, yt-dlp) on macOS; store detected paths via settings.
  • Add helper functions isFFmpegAvailable/isFFprobeAvailable and safer process handling (noThrow) across video utilities and ffprobe/ffmpeg calls.
  • Update video utils and process calls to check exit codes and log failures.

• UI / UX and layout fixes:

  • Replace nested unbounded SingleChildScrollView in main UI with bounded SizedBox/MouseRegion to avoid layout constraints.