Add copy button to generic error dialog

[BenJule]

Summary

This PR adds a non-closing Copy button to the generic ErrorDialog.

Why

Error dialogs often contain details that are useful for bug reports or support requests. A dedicated copy button makes it easier for users to copy the exact error text instead of taking screenshots or manually retyping it.

Related to #11497

Implementation

• Extend MsgDialog::add_button(...) with an optional end_modal argument.
• Keep the default behavior unchanged for existing callers.
• Add a Copy button to ErrorDialog.
• The copy button writes the stored error message to the clipboard and keeps the dialog open.

Scope

This PR only affects the generic ErrorDialog.

It does not:

• add copy buttons to all message dialogs
• change warning, confirmation, or question dialogs
• change error handling behavior
• change the existing OK button behavior

Test plan

• Trigger or instantiate an ErrorDialog.
• Confirm the dialog shows both OK and Copy.
• Click Copy.
• Confirm the error text is copied to the clipboard.
• Confirm the dialog stays open after clicking Copy.
• Click OK.
• Confirm the dialog closes normally.

[Haidiye00]

Hi, please record the effect after the changes, thanks.

[Haidiye00]

Hi, please submit the GIF recording, and merge the patches into one. Thanks.