Add clock mode that displays current system time (#2)

[sarsolot]

Add Clock Mode Feature

🎯 Overview

This PR implements a new clock mode for µTimer that displays the current system time in real-time. The clock mode provides accurate timekeeping by reading the system time every second and displaying it in a clean, readable format.

📋 Changes Summary

🕒 New Clock Mode Feature

• New option: Added --clock (long) and -k (short) command-line options
• Time display: Shows current time in clean HH:MM:SS format (e.g., 14:23:45)
• Real-time updates: Updates every second using system time for maximum accuracy
• System integration: Handles daylight saving time, NTP synchronization, and manual time changes
• User control: Can be stopped using 'Q' key, similar to stopwatch mode

🔧 Technical Implementation

• Timer mode integration: Added TIMER_MODE_CLOCK to existing timer framework
• Clock-specific functions:
  • clock_get_diff(): Reads current system time using GDateTime
  • clock_new_timer(): Creates clock timer instance
  • timer_sec_msec_to_clock_string(): Formats time for clean display
• Conflict detection: Integrated with existing option conflict checking
• Clean architecture: Follows existing code patterns and conventions

📚 Documentation Updates

• README.md: Updated feature list and usage examples to include clock mode
• Manual page (utimer.1): Added clock mode documentation with usage examples
• Help system: Updated command-line help text and descriptions
• ChangeLog: Added detailed feature description and implementation notes

🔧 Code Quality Improvements

• Whitespace cleanup: Fixed trailing whitespace issues in source files
• Version bump: Updated version from 0.5-1 to 0.5-2 for new feature release
• Code consistency: Maintained existing code style and patterns

🚀 Usage Examples

Basic clock mode:

# Show current time (updates every second)
utimer --clock
utimer -k

Example output:

Current Time: 14:23:45 

Integration with existing features:

# Conflict detection works as expected
utimer --clock --timer=5s  # ❌ Shows error about conflicting options
utimer --help              # ✅ Shows new clock option in help

🧪 Testing

• ✅ Clock displays accurate current time in HH:MM:SS format
• ✅ Updates every second with system time
• ✅ Handles time zone changes and system clock adjustments
• ✅ Proper conflict detection with other modes (timer, countdown, stopwatch)
• ✅ Can be stopped with 'Q' key
• ✅ Both short (-k) and long (--clock) options work
• ✅ Help system shows new option correctly
• ✅ All existing modes continue to work unchanged
• ✅ Clean compilation without warnings

📊 Impact

New Capabilities

• µTimer now supports 4 modes: timer, countdown, stopwatch, and clock
• Provides accurate system time display for users who need a CLI clock
• Maintains consistency with existing µTimer interface patterns

Backward Compatibility

• ✅ All existing functionality preserved
• ✅ No breaking changes to existing command-line options
• ✅ Existing scripts and usage patterns continue to work

Performance

• Minimal overhead: one system call per second for time reading
• No impact on other timer modes
• Efficient time formatting for display

🔗 Related Issues

Resolves #2 - Clock mode that shows current time

📝 Notes for Maintainers

• Version bumped to 0.5-2 to reflect new feature
• All source files follow existing code style
• Documentation comprehensively updated
• Ready for release and packaging