refactor(crypto): share byte-compatible Ed25519 signing

[rldyourmnd]

Moves client authentication signing to the same byte-compatible ed25519-dalek implementation as nremote-server PR #14. Persisted 64-byte device keypairs and signature || message payloads remain unchanged.

The signer source is byte-identical in the client and server branches. RFC 8032 vector 1 produces the exact required signature, malformed stored keypairs are rejected, and modified signatures fail verification. sodiumoxide remains only for the separate box_ and XSalsa20-Poly1305 secretbox protocol operations.

This PR is temporarily stacked on #34 and will be retargeted to main once that cleanup merges.

Validation:

• cargo test --locked --all-targets --manifest-path libs/hbb_common/Cargo.toml: 103 passed
• exact signer source parity with nremote-server PR chore(deps): bump tokio from 1.44.2 to 1.46.1 #14
• branding, configuration, and protocol parity gates
• git diff --check