fix: support writable Memmy data on non-system drives

[EF1874]

Summary

• relocate new packaged Windows data roots beside non-system-drive installs
• propagate the resolved Memmy data root across desktop, backend, agent, and Memory services
• validate both install and data-directory writability in interactive NSIS installs
• keep silent updates and existing legacy data behavior unchanged

Verification

• 116 targeted tests passed
• full workspace typecheck passed
• actual unsigned Windows package and final NSIS rebuild passed
• full Memory suite has the same 14 Windows EPERM failures on clean upstream/v1.0.5 (2 symlink permissions, 12 temp SQLite cleanup permissions)