Add Lake Lanier Water Level app

[jspeigner]

Displays current water level data for Lake Sidney Lanier, showing feet above/below full pool (1071 ft) with color-coded status indicators and trend arrows. Includes optional 7-day historical chart view.
Features:
Real-time water level data from USGS
Color-coded status indicators (Green/Yellow/Orange/Red)
Trend analysis (rising/falling/stable)
Historical chart visualization
Schema-based configuration (toggle chart view)
Testing:
App passes all pixlet check validations
Tested locally with pixlet serve
Successfully pushed to test device
Data Source: USGS monitoring station for Lake Sidney Lanier (site ID: 02334400)

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