feat(frontend): complete locale-safe currency and amount formatting

[arisu6804]

Problem

Locale separators and floating-point formatting can cause a transfer amount to differ from the user's intended value.

Objective

Deliver a production-quality improvement to amount inputs, balances, fees, and receipts that creates measurable value for correctness, security, reliability, performance, or maintainability.

Implementation scope

• Use canonical decimal strings for submission; define display precision, currency minor units, rounding, and negative/zero handling.

Acceptance criteria

• Round trips are deterministic across supported locales; unsupported precision is rejected; receipts match the submitted value.

Required validation

• Locale matrix, property round-trip, boundary, and snapshot 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.

[Johnpii1]

hello
pls can i work on this issue please
thanks

[grantfox-oss]

🦊 GrantFox — @Johnpii1 has been assigned to this issue as part of the Third Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #280)
2. Your PR will be reviewed by the RemitFlow maintainers

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