Sync WinUI terminal integration from sftp-explorer

[NikulovE]

Hello!
I use this repo as part of my WinUI 3 application, which includes an integrated terminal: https://apps.microsoft.com/detail/9N6S8V9QQ224

While using it, I added some new features, improved theme support, and fixed several bugs.

[mitchcapper]

Thats fantastic, thanks for the PR will review it shortly. App looks nice as well!

[NikulovE]

Over time, I made a number of changes while integrating and using the terminal in the application. This PR contains those changes mostly as-is, so it also includes some SFTP Explorer-specific customizations that may not necessarily make sense for the upstream project.

Please feel free to clean up the changes, drop anything too application-specific, and keep only the parts you consider useful for this repository.

The most significant improvements in this PR are:

• Terminal resize handling — improved resizing behavior and synchronization with the host control/window.
• Terminal redraw/rendering — fixes and improvements to terminal redraw behavior, especially during resize and UI changes.
• Theme and color handling — improved integration with application themes and terminal colors.
• Resource cleanup — improved disposal and cleanup of terminal-related resources to avoid leaks and resources being kept alive unnecessarily.
• Various smaller fixes and integration improvements discovered while using the terminal in SFTP Explorer.

I hope at least some of these changes can be useful upstream.