[P1] Design authenticated pairing and encrypted transfers

[Gaalbu]

Problem

Protocol v1 relies on user approval and opaque session IDs but sends file content over unencrypted HTTP and does not authenticate peer identity.

Acceptance criteria

• Threat-model an attacker on the same LAN.
• Select reviewed libraries/protocols for pairing, authentication and encryption.
• Define key verification and rotation UX for Android and Linux.
• Preserve explicit approval and avoid custom cryptographic primitives.
• Add interoperability and downgrade tests before changing protocolVersion.