feat(frontend): add locale-safe numeric input and display boundaries

[arisu6804]

Problem

Locale separators, floating-point conversion, and excessive precision can cause users to submit an amount different from what they intended.

Objective

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

Implementation scope

• Parse input using explicit locale rules and canonical decimal strings rather than binary floating point.
• Show supported precision, minimum units, rounding, and insufficient-balance errors before signing.
• Keep display formatting separate from transaction serialization.

Acceptance criteria

• The serialized amount is deterministic across supported locales.
• Invalid, negative, excessive-precision, and boundary values are rejected clearly.
• Displayed values reconcile with the submitted canonical value.

Required validation

• Locale matrix tests, property tests for round trips, and boundary validation 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 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.