Fix CopyPasteSendText using character count instead of byte count

[pwn2ooown]

Summary

CopyPasteUIX11::CopyPasteSendText() in copyPasteUIX11GTK4.cpp uses Glib::ustring::size() as a byte length, but size() returns the character count, not the byte count. This causes multi-byte text (e.g. CJK characters) to be truncated when passed to CPClipboard_SetItem, which expects a byte length.

Fix: Replace cpStr.size() with cpStr.bytes() to get the correct byte count.

Fixes #800

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[pwn2ooown]

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