Add focused native Picture-in-Picture support

[DavidAGInnovation]

Summary

• handle the win-set-pip / win-pip-changed IPC used by stremio-web#1354
• reparent the existing MPV child into a resizable, always-on-top native window
• resize MPV with the PiP window and restore it safely when PiP closes
• persist the PiP window placement
• add a tray toggle as a fallback control

Review notes

This is a focused replacement for #78. It keeps PiP shell-side, removes the duplicated web-control injection, avoids a custom GDI+ overlay, and does not guess at unrelated child windows when locating MPV. The web control remains owned by stremio-web#1354.

Validation

• cargo fmt --all -- --check
• cargo check --target x86_64-pc-windows-msvc (passed; the macOS host required skipping only the Windows resource compiler step)
• git diff --check with the repository's CRLF files handled