fix: document contract error codes and map backend messages

[MickeeJay]

docs/feat: Document contract error codes and add backend error mapping

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Summary

Contract error codes were undocumented and incompletely surfaced. The backend
returned raw Soroban errors without decoding contract-specific codes, leaving
API consumers with no actionable information on failure. This PR closes both
gaps with a complete error code reference and a backend mapping layer.

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Root Cause

Two independent gaps compounded the problem:

• The error code table in documentation was incomplete, with no stability
  guidance or trigger conditions described per code
• Backend contract-invoking routes surfaced raw Soroban errors directly,
  with no decoding step to translate contract error codes into user-facing
  messages

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###Changes made

1. Error code documentation
Added a complete error code reference table covering:

Column	Detail
Error code	Canonical contract error identifier
Trigger condition	When and why the contract emits this error
Stability guidance	Whether the code is stable, provisional, or subject to change

2. Contract error mapping helper
Implemented a dedicated helper that decodes raw Soroban contract error codes
into structured, user-facing messages.

3. Route integration
Applied the mapping helper across all contract-invoking backend routes,
replacing raw Soroban error passthrough with decoded responses.

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Impact

Area	Before	After
Error code documentation	⚠️ Incomplete	✅ Complete with stability guidance
Backend error surface	❌ Raw Soroban errors	✅ Decoded user-facing messages
Contract-invoking routes	❌ Unhandled passthrough	✅ Mapped via shared helper

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Closes #58

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