fix: suggest prefixed units for unknown identifiers - #903

Closes #103

Summary

• Include valid prefixed units in unknown-identifier suggestions.
• Respect metric/binary prefix support and short/long alias settings.
• Prefer MiB over unrelated suggestions when the user enters Mib.
• Preserve existing suggestions for ordinary unit names.

Tests

• Add regression coverage for Mib, kiolmeter and niles.
• cargo test -p numbat --quiet
• cargo fmt --all -- --check
• git diff --check
• cargo clippy -p numbat --all-targets -- -D warnings -A clippy::map_or_identity

Clippy excludes only the existing map_or_identity warning in numbat/src/list.rs:94, which is present on upstream main and is unrelated to this change.