feat(frontend): add preflight simulation before value-moving transactions

[arisu6804]

Problem

Users can reach the wallet-signing step with stale balances, paused contracts, unsupported amounts, or provider conditions that would make a value-moving transaction fail.

Objective

Add a safe, read-only preflight step that catches deterministic failures before requesting a signature while preserving the authoritative on-chain result.

Implementation scope

• Run the configured simulation or validation endpoint against the current wallet, network, and transaction payload.
• Present actionable failure reasons and invalidate the preflight when relevant state changes.
• Never treat a successful simulation as confirmation and never bypass contract-side validation.

Acceptance criteria

• Deposits and withdrawals do not request a signature when preflight detects a deterministic failure.
• Preflight results are tied to the exact serialized transaction and connected network.
• Provider timeouts and unsupported simulation responses produce a safe retry path.

Required validation

• Add tests for success, contract rejection, timeout, stale balance, network switch, and changed payload.
• Existing tests and CI remain passing.
• Do not weaken, delete, or skip unrelated tests to obtain a green build.

PR quality bar

• Explain the simulation trust boundary, race handling, user-visible states, and test evidence.
• Keep the PR focused; avoid unrelated UI polish or documentation-only changes.

Out of scope

• Replacing authoritative contract validation or redesigning wallet integrations.