test(frontend): add contract-driven UI tests for API errors and precision

[arisu6804]

Problem

Frontend regressions are likely when API error codes, transaction states, or decimal fields change without a consumer-level check.

Objective

Deliver a production-quality improvement to API client adapters and core vault screens that creates measurable value for correctness, security, reliability, performance, or maintainability.

Implementation scope

• Generate or maintain fixtures for validation, authorization, provider, pending, and terminal errors.
• Assert parsing of precision-safe numeric fields and pagination/correlation metadata.
• Exercise deposit, withdrawal, and position views against the contract fixtures.

Acceptance criteria

• Incompatible API shapes fail tests with actionable diagnostics.
• The UI renders each supported error state without leaking raw responses.
• Tests run deterministically without live wallet or chain dependencies.

Required validation

• Contract fixtures, component tests, and deterministic integration tests for all listed states.
• Existing tests and CI remain passing.
• Add regression coverage for the original failure mode.
• Do not weaken, delete, or skip unrelated tests to obtain a green build.

PR quality bar

• Keep the PR focused on this issue and explain design tradeoffs.
• Include test evidence, compatibility impact, and migration or rollout notes where relevant.
• Avoid typo-only, documentation-only, cosmetic-only, or unrelated refactor submissions.

Out of scope

• Broad rewrites not required by the acceptance criteria.
• Changes to unrelated services, contracts, or user flows.