stellarErrorParser: Add human-readable mappings for common Stellar error codes

[Kingsman-99]

Description

stellarErrorParser extracts error codes from Stellar transaction results but returns raw codes (e.g. "op_no_trust") without explanation. Contributors should add a mapping table that converts known Stellar operation result codes to user-friendly messages (e.g. "The recipient does not have a trustline for this asset") so error toasts are actionable.

Acceptance Criteria

• A mapping covers at minimum 15 common Stellar operation result codes with clear English messages
• Unknown codes fall back to a generic message: "Transaction failed: {raw code}"
• The function returns both { code, message } so callers can use the raw code for logging
• Existing tests continue to pass

Context

• Target file: src/lib/stellarErrorParser.ts
• Error display: src/lib/errors.ts