[FEATURE]: Copy selection as markdown in Desktop app #54002

Feature hasn't been suggested before.

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Describe the enhancement you want to request

In the Desktop app, selecting a portion of an assistant message and copying it yields rendered plain text with all markdown structure stripped. #41609 asks for whole-message raw markdown copy, but that is TUI/CLI-focused — the Desktop app has no equivalent for either whole-message or selection copy.

Request: when text is selected in the Desktop chat transcript, the copy action should place the markdown source for that selection on the clipboard instead of rendered plain text.

Notes:

• Selections frequently cut across block boundaries (half a paragraph, part of a table, a few list items, mid-code-block). The implementation should decide a deterministic behaviour for partial blocks, e.g. re-emit the enclosing block as markdown, or fall back to plain text for the partial portion.
• Related: [FEATURE]: Copy message as raw markdown #41609 (per-message raw markdown copy in the TUI).

Use case: quoting a specific snippet from a longer response into an issue, PR description, or another AI prompt without losing code fences, links, and list structure.