feat(frontend): prevent duplicate transfer submissions across retries and navigation

[arisu6804]

Problem

A double click, refresh, or provider timeout can submit the same transfer more than once.

Objective

Deliver a production-quality improvement to send form, mutation state, and navigation recovery that creates measurable value for correctness, security, reliability, performance, or maintainability.

Implementation scope

• Bind a canonical request fingerprint and idempotency key to the exact payload; persist only a safe operation reference; reconcile after navigation.

Acceptance criteria

• One user intent creates at most one transfer; refresh restores status; conflicting edited payload requires a new intent.

Required validation

• Double-click, refresh, timeout, navigation, and changed-payload integration tests.
• 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 and explain design tradeoffs, compatibility impact, and test evidence.
• 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.