Replace custom character whitelist with Unicode standards-based validation:

- Use unicode.IsLetter/IsNumber/IsSymbol instead of hardcoded lists
- Remove manual UTF-8 byte checking for CJK punctuation
- Add proper rune-based length limiting (MAX_TAG_LENGTH = 100)
- Improve international character support (CJK, Arabic, Cyrillic, etc.)
- Add emoji support via unicode.IsSymbol

Benefits:
- Cleaner, more maintainable code (~50 lines removed)
- Standards-based approach following Unicode categories
- Better UTF-8 safety with utf8.DecodeRune
- Consistent validation between Go backend and TypeScript frontend

All existing tests pass with improved Unicode handling.