feat(frontend): implement client-side Soroban transaction simulation sandbox — dry-run before signing, fee breakdown, storage state diff, risk indicators, and pre-sign confirmation modal

[Chibey-max]

Overview

Users currently sign transactions blindly — they see the Freighter pop-up but have no understanding of what the transaction will do, what state the contract will be in after it succeeds, or how much it will cost. A simulation sandbox runs the transaction against testnet before the user signs, showing them the exact outcome.

What to build

• SorobanSimulator service (frontend, lib/soroban-simulator.ts):
  • Accepts a built (unsigned) SorobanTransaction XDR
  • Calls simulateTransaction on the configured Stellar RPC endpoint
  • Parses the SimulateTransactionResponse: resource fee, CPU instructions, memory bytes, read/write ledger entries
  • Returns { success: bool, fee_estimate: { resource_fee, base_fee, total_fee_xlm }, state_diff: LedgerEntryDiff[], error?: string }
• LedgerEntryDiff — human-readable diff of storage entries that would change:
  • Parse the readWrite footprint from the simulation response
  • Decode each LedgerEntry XDR using the contract's ABI (fetched from the contract ABI registry, feat(backend): build Soroban contract ABI version registry — deployed WASM hash tracking, method signature changelog, compatibility matrix, and REST API for client version negotiation #1530)
  • Display as { key, old_value, new_value } where old value is fetched from chain and new is from simulation
• TransactionPreviewModal component (shown before any sign() call):
  • "What this transaction does" description (derived from the method name and decoded arguments)
  • Fee breakdown table: resource fee, base fee, total in XLM and USD (using live rate from full-stack issue)
  • State diff panel: collapsible table of storage changes
  • Risk indicators: warn if transaction writes to more than 10 storage keys (large footprint), or if resource fee exceeds 1 XLM
  • "Sign & Submit" and "Cancel" buttons
• Wire into: escrow creation, milestone release, dispute filing — every Soroban transaction entry point

Acceptance Criteria

• Simulation calls use the RPC simulateTransaction endpoint; not submitTransaction
• Fee is displayed in both XLM (7 decimal places) and USD (2 decimal places via live rate)
• State diff correctly decodes at least: EscrowState, MilestoneState, u128 amounts
• Simulation failure (contract error) is shown with a human-readable error message, not raw XDR
• Modal blocks signing until the user explicitly clicks "Sign & Submit"
• Cancel discards the unsigned transaction; no state mutation occurs
• Loading state while simulation is in-flight; spinner + "Simulating transaction..." label
• Unit tests for SorobanSimulator.parseFee and LedgerEntryDiff decoding with fixture XDR blobs

Technical notes

• Use @stellar/stellar-sdk rpc.Server.simulateTransaction() for simulation
• ABI decoding: parse ScVal XDR using the contract's method schema from the registry; unknown types fall back to raw hex display