docs(error-handling): add error handling conventions doc (#630)

[omoh5]

Summary

Adds docs/error-handling-conventions.md, the single source of truth for how every error category is surfaced in the Access Layer client (toast, inline, section boundary, page boundary, transaction surfaces, network banner).

Also adds a cross-link from docs/state-management.md section 3 (Error Architecture Strategy) to the new conventions doc.

Closes #630.

Acceptance Criteria Coverage

• Query error handling conventions documented with decision criteria — Mode 2 (Inline) flowchart, ApiError.status cheat sheet, decision flowchart at top of doc
• Mutation error display patterns documented with examples — Mode 1 (Toast) with branches for status 0 / 4xx / 5xx / 422 validation, plus user-rejection (getSignatureErrorMessage) and off-chain mutation patterns
• New error type addition process described — Adding a New Error Type section covers WALLET_ERROR_COPY extension in src/utils/errorHandling.utils.ts AND the typed union + Record table pattern in src/utils/claimActionDisabledReason.ts
• Shared error state component referenced with usage example — Mode 2 uses CreatorProfileErrorState, Mode 3 uses SectionErrorBoundary, Mode 4 uses CreatorPageErrorBoundary and AppErrorBoundary, Mode 5 uses TransactionRetryNotice + TransactionFailureDrawer. Each reference includes a code example and the component prop contract.
• Document lives in docs/ and cross-links the state management conventions doc — docs/error-handling-conventions.md links prominently from the top (intent line) and from the Cross-references section; docs/state-management.md section 3 now points to it

Changes in this PR

File	Change
docs/error-handling-conventions.md	New file covering the full convention
docs/state-management.md	Cross-link sentence in section 3 Error Architecture Strategy

Why now

Error handling across the client has been inconsistent — some pages use error boundaries, some handle query errors inline, and mutation errors are displayed differently depending on the component. This doc establishes a single decision flowchart (and per-mode reference) so every future component lands in the right bucket.

Cross-references inside the doc

• State Management section 3 Error Architecture Strategy (boundary vs inline)
• Error Handling in React Query Hooks (ApiError branching detail)
• API Layer Conventions (BaseApiService.handleError source)
• Shared Components (toast/skeleton/empty-state families)
• Adding a Page and Data Fetching (end-to-end route wiring with boundaries)
• BuyActionHelperText and ClaimActionHelperText docs (pre-action copy system)

Verification

• pnpm exec prettier --check docs/error-handling-conventions.md docs/state-management.md -> no style issues
• No code or test changes — docs only.

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