Fix Windows desktop user-directory resolution

[Ghost-Frame]

Summary

• resolve default data and config roots with platform-native user directories
• preserve non-empty XDG overrides on Linux
• fail closed when the OS provides no user-specific directory
• add regression coverage for GUI environments without HOME

Verification

• cargo test -p frameshift-client (158 tests across unit and integration targets)
• cargo fmt pre-push gate
• cargo audit pre-push gate