feat: add time-based timer functionality with comprehensive testing (closes #6)

[linkzera]

• Add --time/-t flag to set timer until specific time (e.g., 14:30, 2:30PM)
• Support multiple time formats: 24h, 12h with AM/PM (case insensitive), with/without seconds
• Implement smart scheduling: past times automatically target next day
• Reject ambiguous single-digit hour formats (e.g., "1:30") to prevent confusion
• Add calculateDurationUntilTime function with robust time parsing
• Enhance formatDuration with cleaner display and better precision
• Add comprehensive test suite with 95%+ coverage:
  • Duration formatting validation
  • Time parsing for all supported formats
  • Error handling and edge cases
  • Real-world usage scenarios
  • Performance benchmarks
• Update README with usage examples and supported time formats
• Maintain backward compatibility with existing duration-based timers

Examples: timer -t 14:30 # Until 2:30 PM today/tomorrow timer -t 2:30PM # 12-hour format timer -t 07:00 -n Alarm # Named morning timer