Implements FR-020: Grace Time (buffer period after free time)
Problem: Fan relay signal continues ~5s after laser stops, causing
false billable time charges when user stops at countdown 1 second.
Solution: New GRACE state between GRATIS and NET_COUNTING with
configurable grace period (default 5s, range 0-30s). During grace
time, display shows blinking 0 (number of blinks = remaining seconds).
If laser stops during GRACE, no billable session is created.
State machine: INACTIVE -> GRATIS -> GRACE -> NET_COUNTING
Changes:
- config.h: Add DEFAULT_GRACE_TIME_S (5), MIN/MAX constants
- settings.h/cpp: Add graceTimeSeconds field, NVS persistence
- laser_tracker.h/cpp: Add GRACE state, transition logic
- display_manager.h/cpp: Add showGraceBlinking() with blink animation
- main.cpp: Add GRACE state handling in display update loop
- mqtt_client.cpp: Add grace_time_s field to session JSON
- web_server.cpp: Add Grace Time input field in config page
- README.md: Update documentation with GRACE state details
- Feature-Requests.md: Mark FR-020 as completed (v1.6.4)
Version: 1.6.4
Tested: Manual testing with logs and MQTT Explorer
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