[FEATURE]: Add support for Linux PRIMARY selection (middle-click paste)

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Describe the enhancement you want to request

Description

Currently, the TUI only targets the CLIPBOARD selection when copying text. On Linux, this means that while Ctrl+C/V works, the native "highlight to copy" and "middle-click to paste" behavior (PRIMARY selection) is not supported.

This makes the TUI feel non-native on Linux environments.

Proposed Solution

Introduce a new configuration option linux_clipboard_selection in the TUI config with the following values:

• clipboard: Standard Ctrl+C/V (current behavior)
• primary: Targets the middle-click buffer
• both: Updates both buffers simultaneously (recommended default)

This requires using xclip, xsel, or wl-copy to explicitly target the primary selection.

Copied from issue #29963