[FEATURE] Desktop: option to copy thinking blocks as plain text

Problem

In the desktop app (Code tab), the copy button on an expanded thinking block copies it as a Markdown blockquote with a header:

> **Thinking** (~36 tok)
>
> I'm looking for audio-related terms like tts, voice, wav, ffmpeg, and audio in the context, since most of this seems to be context compression.

I usually paste thinking into notes or back into a prompt, so every time I have to strip the header line and the > on each line by hand. On a long block that's a lot of lines.

Request

A copy-format preference in Settings, so the button can copy just the text:

I'm looking for audio-related terms like tts, voice, wav, ffmpeg, and audio in the context, since most of this seems to be context compression.

Related: #54670 asks for the opposite (Markdown source) in the VS Code extension, so I think a setting fits better than changing the default.

Platform: Windows 11, desktop app, Code tab