fix: improve labeledNumber formatting - handle edge cases, avoid mutation

[liuyikai1103]

Fixes #11

Changes

1. Avoid mutation: uses local absNum instead of modifying input
2. Handle negative numbers: correctly preserves the sign
3. Handle 0 and undefined: returns proper values
4. Cleaner format: no trailing .0 for round numbers (e.g. 1K not 1.0K)