feat: use different application icon for debug builds

[JC-Chung]

What

Use a distinct icon (AppDev.ico) for non-Release builds so debug and release binaries can be easily told apart in the file explorer and taskbar — including when both versions are pinned to the taskbar.

Changes

• src/SourceGit.csproj:
  • ApplicationIcon is now conditional (App.ico for Release, AppDev.ico otherwise)
  • DEV_BUILD constant is defined for non-Release builds
  • Both App.ico and AppDev.ico are included as AvaloniaResource
• src/App.axaml.cs: Apply AppDev.ico to all windows at startup via Window.WindowOpenedEvent class handler when DEV_BUILD is defined
• src/AppDev.ico: Color-adjusted version of App.ico (hue modified using GIMP). Feel free to replace with a custom design.

The official Release build (-c Release) is unaffected.