feat: map Soroban simulation failures to human-readable BetModal errors (#406)

[meem08]

Closes #406

Summary

Translates common Soroban simulation and Freighter signing errors into player-friendly copy shown in the BetModal — both the fee-estimate failure panel and the transaction failure timeline — so players see actionable messages instead of raw SDK / HostError internals. The raw error is always kept in the console for debugging.

Changes

• src/lib/xelma-contract.ts — new exported humanizeContractError(error, context?):
  • 9 mapped error cases: insufficient balance, Soroban budget/resource exceeded, closed round, user-cancelled Freighter signature, signing failure, authentication failure, unfunded account, network/timeout, plus a generic contract-rejection catch-all.
  • Unknown errors fall back to a safe generic message (CONTRACT_ERROR_FALLBACK).
  • Raw error (message + stack) is logged to the console with a [xelma-contract:<context>] prefix for debugging.
• src/components/BetModal.tsx — uses humanizeContractError for both the fee-estimate failure message and the transaction failure timeline (tx.fail(...)).
• Tests — added a humanizeContractError unit-test suite in src/lib/__tests__/xelma-contract.test.ts (mapped cases, fallback for unknown/null errors, raw-error console logging) and updated the two BetModal test files' mocks/assertions to expect the friendly copy.

Acceptance criteria

• At least 5 mapped error cases (9 mapping rules implemented)
• Unknown errors show a safe fallback message
• Raw error kept in console for debugging

Verification

• pnpm exec tsc -b — passes
• pnpm build — passes
• pnpm test:unit — 791 tests pass
• ESLint clean on all changed files (pre-existing react-hooks/set-state-in-effect errors in PredictionHistory.tsx / PriceChart.tsx / Dashboard.tsx / Learn.tsx are unrelated to this change)

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