Unify file transfer recipients with userlist

[ctoth]

Summary

• replace the raw file-transfer recipient text path with an editable dropdown backed by the MCP userlist
• add a file-transfer peer adapter for selecting known users and resolving incoming sender labels
• keep protocol sends string-based while restricting the UI send action to selected connected players
• add focused tests for peer mapping and dropdown selection behavior

Verification

• npm test -- src\fileTransferPeers.test.ts src\components\FileTransfer\Controls.test.tsx
• npm run build