Harden secure wallet storage failure handling

[comzzy-comzzy]

Summary

• centralize safe secure-storage error messages and recovery guidance
• validate and sanitize wallet secrets on every secure-store read before returning them
• update wallet save, import, export, send, review, vault, signer, and reset flows to avoid crashes or vague secret-key errors
• add focused coverage for malformed stored secrets during secret-key access

Tests

• npm test -- --runInBand tests/walletStore.test.ts tests/walletStore.test.ts
• npm run typecheck (fails: existing tsconfig customConditions requires moduleResolution node16/nodenext/bundler)

Closes #257