docs: unify peer-oriented terminology

[YoungJinJung]

Closes #24

Summary

• align README wording around peer, share code, and send/receive
• update command help and prompt text to use the same vocabulary
• refresh tutorial wording so onboarding language matches the rest of the CLI

Validation

• go test ./cmd/ende ./internal/...

[amazon-q-developer]

This PR successfully unifies the terminology across the codebase to be more peer-oriented, replacing technical terms like "recipient", "sender", and "token" with user-friendly terms like "peer" and "share code". The changes are consistent across documentation, command descriptions, prompts, and tests. All code continues to function correctly with no logic errors or functional defects introduced.

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