Migrate spatial backend from {raster} to {terra}

The goal of this task is to replace the legacy {raster} package with the modern {terra} package to improve performance, memory management, and long-term maintainability of the application.

Key Changes

• Data Loading (global.R): Replace raster::brick() with terra::rast() and update CRS assignment using terra::crs()
• Spatial Operations (server.R):
  • Replace raster::crop() and raster::extent() with terra::crop() and terra::ext()
  • Switch from raster::rasterToPolygons() to terra::as.polygons()
  • Note: Use aggregate = FALSE in as.polygons() to maintain 1:1 parity with the legacy pixel-based grid
• Coordinate Transformation: Standardize on sf::st_transform() for WGS84 (EPSG:4326) projections required by Leaflet

Implementation Logic

To ensure a safe transition, a side-by-side validation was performed using suffix 2 variables (e.g., X12, raster_tmp2) to compare:

1. Bounding Boxes: Ensuring perfect geographic alignment
2. Extracted Values: Verifying that parameter values match between packages

Benefits

• Significant speed increase in polygon generation
• Reduced memory footprint during cropping operations
• Compatibility with modern R-Spatial standards