feat: add identicons to signer management list

[LockedTerminal]

Closes #451

• Add frontend/src/utils/identicon.ts: pure client-side deterministic identicon generator using FNV-1a 32-bit hash, no network calls. Produces a symmetric 5×5 pixel-grid SVG with address-derived colour.
• Update SignerRow in SignerManagement.tsx to render an component alongside each signer address for at-a-glance recognition.
• Add .signer-identicon CSS class and .signer-td__address-wrap flex container so the icon and address sit inline cleanly.
• Add SignerManagement.test.tsx asserting:
  • one identicon rendered per signer
  • accessible aria-label includes shortened address
  • distinct SVGs for different addresses
  • same address always yields the same SVG (determinism)

Checklist

• Closes #__
• Tests added or updated
• ABI snapshot updated if contract sources changed (abis/ and COMEBACKHERE-contracts/)
• Screenshots attached if UI changed
• Documentation updated if needed

Notes

• For contract changes, verify ABI snapshot hygiene with make check-abi-snapshots.
• For UI changes, include screenshots or screen recordings to help reviewers.

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