fix: align DSH activation with Mnemon store lifecycle

[Grivn]

Summary

• preserve Mnemon’s required native Store lifecycle while allowing DSH to activate zero or multiple Memory Spaces
• create the first Store as default, prevent deleting the last native Store, and switch native selection before removal when needed
• repair native selection after Pack import and expose Mnemon default versus DSH activation separately
• clarify the distinction in the UI and bilingual documentation without changing the existing activation terminology

Verification

• pnpm run verify
• targeted service/client tests after the final test additions
• manually verified removal of a DSH-inactive Memory Space against Mnemon 0.2.3 after reloading the Host plugin