fix: avoid set_env panic on Windows doctor

[justjavac]

Summary

• Avoid set_env on Windows in dvm doctor, since it can panic while touching PowerShell profile files
• Use PowerShell/.NET user environment APIs on Windows for DVM_DIR and Path
• Move the dvm bin path to the front of user and process PATH without duplicating entries
• Keep the existing set_env behavior for non-Windows platforms

Fixes #222

Tests

• cargo test
• Added a Windows unit test for PATH de-duplication and front insertion

Note: I did not run dvm doctor directly because it writes persistent user environment variables.