[Code scan] Include the GPLv3 companion text for LGPLv3 licensing

This issue was found during a Codex global code scan of the repository.

Baseline commit: e3c5b38a99eb7ab778b5d8c68a5ed4ddf6cf91b3

Problem

The repository contains the LGPLv3 text in `LICENSE`, but the LGPLv3 text incorporates and refers to the GNU GPL. A separate GPLv3 companion text was not present in the tracked files.

Code references:
https://github.com/deepmodeling/dpgui/blob/e3c5b38a99eb7ab778b5d8c68a5ed4ddf6cf91b3/LICENSE#L9-L11
https://github.com/deepmodeling/dpgui/blob/e3c5b38a99eb7ab778b5d8c68a5ed4ddf6cf91b3/LICENSE#L76-L77
https://github.com/deepmodeling/dpgui/blob/e3c5b38a99eb7ab778b5d8c68a5ed4ddf6cf91b3/LICENSE#L91-L92

Impact

This is license hygiene rather than a runtime failure. Downstream redistributors and source package consumers may expect both GPLv3 and LGPLv3 texts to be present when the project is licensed under LGPLv3.

Suggested fix

Add the GPLv3 text as `COPYING` and keep the LGPLv3 text as `COPYING.LESSER`, or use another conventional file layout that includes both license texts in source and release artifacts.