Add native Countdown app

[kelaocai]

Summary

• Adds a native Countdown app configurable through COUNT, CDATE, CICON, and COUNT_COL.
• Shows remaining calendar days before the target date, shows only the configured icon on the target date, and automatically disables Countdown after the target date has passed.
• Adds settings/API documentation and a small utility test for countdown date parsing and display-state behavior.

Test Plan

• c++ -std=c++11 -Isrc test/countdown_utils_test.cpp src/CountdownUtils.cpp -o countdown_utils_test
• ./countdown_utils_test
• pio run
• pio run -e awtrix2_upgrade

[kelaocai]

Here's a screenshot of the local web dashboard I built with the API. It provides a simple interface for configuring and controlling the countdown timer.