Fix terminal state cleanup on application exit

[Aaronontheweb]

Summary

Fixes #44 - Terminal state not fully cleaned up on application exit.

Ensure terminal is fully restored when Termina application exits by adding missing cleanup steps to RunAsync() finally block:

• DisableMouse() - turn off mouse tracking if enabled
• ResetColors() - emit \x1b[0m to reset text attributes
• Flush() - actually send buffered ANSI commands to terminal
• Console.WriteLine() - clear any partial line artifacts

Before

Users had to manually reset terminal state after Termina shutdown:

Console.Write("\x1b[?25h\x1b[0m");
Console.WriteLine();
Console.ResetColor();

After

Terminal state is automatically restored - no manual cleanup needed.

Test plan

• All 561 existing tests pass
• Manual test: Run demo app, exit with Ctrl+Q, verify no artifacts
• Manual test: Run demo app, exit with Ctrl+C, verify no artifacts