Migrate proving system from UltraPlonk to UltraHonk

[Huygon764]

Problem: Currently using UltraPlonk which is deprecated. UltraHonk is the recommended replacement.

Context: Previously attempted UltraHonk but hit a compilation error. Same code/circuit worked with UltraPlonk so it was used to unblock progress. The exact error is no longer recalled.

Expected outcome: Replace UltraPlonk with UltraHonk across the stack (circuit compilation, proof generation in frontend, proof verification in backend/contract). All existing functionality (private transfer deposit/withdraw) works the same.

Notes: Circuit logic should remain unchanged — only the proving system and verification layer need to be swapped.