fix: keep transfers awake and center Windows window

[St0ff3l]

Summary

• Center the restored Windows main window before it is first shown.
• Hold a shared OS idle-sleep inhibition lease throughout file transfer runs; allow display sleep and release after the last run.
• Document the transfer power boundary, regression checklist, and keepawake license.

Validation

• 652 Rust library tests pass, including six transfer power tests and the Windows native execution-state check.
• Tauri typecheck and Windows production build pass.
• Local strict lint/format checks are affected by CRLF checkout behavior; normalized Prettier and ESLint without its formatting rule pass.
• Strict Clippy reports existing warnings in unrelated code; documented in the completed plan.