feat(frontend): implement API client layer for Axum backend with retry, timeout, and signature auth (#801)

[D-Lord-15]

Summary

Closes #801

Implements the frontend API client layer to communicate with the Axum backend for inheritance plan management.

Changes

New Files

• frontend/app/lib/api/inheritance.ts — InheritanceAPI client with handlers for all 5 inheritance endpoints:
  • createPlan() — POST /api/plans (signature auth)
  • getPlans() — GET /api/plans (public, with owner/beneficiary filters)
  • pingPlan() — POST /api/plans/ping (signature auth)
  • triggerPayout() — POST /api/plans/payout (signature auth)
  • getPayoutStatus() — GET /api/anchor/payout-status (public, paginated)
• frontend/tests/api/inheritance.test.ts — 18 unit tests covering success, validation errors, and auth failures

Enhanced Files

• frontend/app/lib/api/client.ts — Enhanced ApiClient with:
  • Configurable request timeout via Promise.race (30s default)
  • Exponential backoff retry with full jitter (3 retries, 408/429/5xx)
  • Ed25519 signature auth (X-Public-Key + X-Signature headers)
  • Per-request RequestConfig overrides (timeout, retry, headers)
  • Clean TypeScript interfaces: RetryConfig, RequestConfig, SignatureAuth
• frontend/app/lib/api/index.ts — Exports new types and InheritanceAPI singleton
• frontend/tests/mocks/handlers.ts — MSW handlers for /api/plans and /api/anchor/payout-status with validation logic and pagination

Architecture Decisions

• Timeout: Promise.race-based timeout avoids jsdom/MSW AbortSignal class incompatibility in test environments
• Retry: Exponential backoff with full jitter, configurable per-request
• Auth: Dual auth — JWT Bearer for admin, ed25519 signature for Stellar wallet users (X-Public-Key + X-Signature)
• DTOs: Clean TypeScript interfaces mapping 1:1 to backend Rust response types

Testing

npx vitest run tests/api/inheritance.test.ts
# 18/18 tests passing

Coverage includes:

• Plan creation with validation (400 errors, allocation_bps sum check, empty beneficiaries)
• Plan fetching with owner/beneficiary filters
• Plan pinging with auth (401 on invalid signature, 404 on missing plan)
• Payout triggers
• Payout status with pagination and beneficiary filtering
• Server error scenarios (500)

Type Check

npx tsc --noEmit  # passes clean

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[ONEONUORA]

@D-Lord-15
Pls make implementation to pass CI checks