Fix invalid amount parsing and reject unknown output currencies

[gcomte]

Empty amounts and multibyte SI suffixes could crash the CLI, while non-finite amounts could print NaN or infinity with a successful exit status. A misspelled output currency could also unexpectedly fall back to the configured fiat conversions.

Parse SI suffixes at UTF-8 character boundaries, reject empty/non-finite/overflowed amounts and non-finite configured defaults, and report invalid explicit output currencies as usage errors with no stdout. Valid SI notation, separators, scientific notation, and omitted-argument defaults remain supported.

Validation:

• All 28 tests pass, including the existing ignored table test and live API tests.
• Four new regression tests cover invalid amounts, multibyte SI suffixes, supported numeric formats, and rejecting invalid outputs before fetching rates.
• cargo fmt --all --check and cargo clippy --all-targets -- -D warnings pass.
• Post-implementation security review found no actionable findings.

This is the base PR in the input-validation, exact-arithmetic, and CLI-output stack.

[coderabbitai]

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Run ID: f633cd26-1f5e-445a-a60c-693e57505afb

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