Fix GDI resource leaks, disposal pattern, and minor code quality improvements

[Copilot]

The codebase had GDI handle leaks in icon creation, no resource teardown path on exit, and several minor quality issues.

Resource leaks

• Icon creation: Bitmap was never disposed; Icon.FromHandle() does not take ownership of the raw HICON. Fixed by using using Bitmap, cloning the managed Icon, then releasing the HICON via a new DestroyIcon P/Invoke:

  IntPtr hIcon = bmp.GetHicon();
  Icon icon = (Icon)Icon.FromHandle(hIcon).Clone();
  DestroyIcon(hIcon);
  return icon;

• Exit cleanup: duplicated manual teardown replaced by a proper Dispose(bool disposing) override on ApplicationContext that unhooks the keyboard hook, disposes the StreamWriter, NotifyIcon, and both icons. A _disposed guard prevents double-disposal.

Hook callback

• Added WM_SYSKEYDOWN / WM_SYSKEYUP (0x0104/0x0105) handling so Alt-key combinations are captured.
• Added writer != null guard to prevent NullReferenceException if log-file creation fails.

Project file

• Switched SDK from deprecated Microsoft.NET.Sdk.WindowsDesktop to Microsoft.NET.Sdk.
• Enabled <Nullable>enable</Nullable> to surface nullability issues at compile time.

Minor

• stopwatch, keyDownTimes, _proc marked readonly.
• Removed redundant releaseTime intermediate variable.
• Replaced nested using statements with modern using declarations.
• Removed emoji from inline comments.

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