feat(web): map simulate errors to ErrorBanner via parseContractError (#561)

[davismas23]

Summary

Closes #561

Wires contract simulation failures through the existing parseContractError helper and surfaces them in both the inline error alert and the global toast (ErrorBanner) so users see a short, human-readable message rather than a raw Soroban SDK error string.

Problem

queryContract (used by every read-only call including getPosition) was re-throwing raw errors. PositionPanel's catch block forwarded err.message directly without mapping it through parseContractError, and had no toast notification at all.

A Soroban simulation error like:

Simulation failed: Error(Contract, #6) ...

was displayed verbatim. After this PR it becomes:

Deposits are currently closed for this market.

Changes

apps/web/lib/contract-client.ts

queryContract now parses every thrown error through parseContractError before re-throwing:

catch (error) {
  console.error('Contract query error:', error);
  const { parseContractError } = await import('@/lib/errors');
  throw new Error(parseContractError(error));
}

• Contract error codes → human copy from MARKET_ERROR_MESSAGES
• Simulation failed: ... → Transaction would fail: ...
• Wallet rejections → Request was rejected in the wallet.
• Unknown errors pass through unchanged

The import is dynamic (same pattern as the Freighter import in invokeContract) so this module stays safe on server-rendered paths.

apps/web/components/PositionPanel.tsx

• Imports useToast from @/context/ToastContext
• Imports parseContractError from @/lib/errors (defensive passthrough if already parsed)
• useToast() destructured in the component body
• showToast added to the useCallback dependency array
• catch block now:

  const reason = parseContractError(err);
  setError(reason);           // inline role="alert" block
  showToast(reason, 'error'); // global toast — auto-dismisses after 6 s

• Updated JSDoc to document the simulation-error surfacing behaviour

Error flow after this PR

getPosition()
  └─ queryContract()
       └─ simulateTransaction()     ← throws on simulate failure
            └─ parseContractError() ← maps to user-facing copy
                 └─ throw new Error(reason)
PositionPanel.refresh() catches:
  const reason = parseContractError(err)  ← already parsed, passthrough
  setError(reason)     →  inline <p role="alert">
  showToast(reason)    →  global toast (ErrorBanner, 6 s auto-dismiss)

Acceptance criteria

• Failed simulate shows recoverable message (inline alert + toast)

Notes

• PR is focused on issue Contract client maps simulate errors to ErrorBanner #561 only
• No contract changes
• No new dependencies — uses existing useToast / parseContractError