Clearer MeshNode menu labels

The mesh menu next to the search bar now labels its actions Create MeshNode, Import MeshNode, and Export MeshNode, so it is clear they operate on mesh nodes rather than uploading or downloading arbitrary files.

The top-bar menu buttons also read consistently on hover: the AI button now shows "AI menu" and the GitHub button "GitHub menu", matching the Node and Mesh buttons. All labels are available in English and German.