Add BITS transfer priority configuration support

Introduces a new parameter to control BITS download priority across the build system and UI, allowing users to optimize transfer speeds when needed.

The feature adds a priority selector to the UI with four options (Foreground, High, Normal, Low) and propagates the selection through the build script and common modules. Priority can be set via UI, command-line parameter, or environment variable, with Normal as the default.

Updates the BITS transfer retry logic to respect the configured priority instead of hardcoding Normal priority, and fixes minor code formatting inconsistencies.

FFUDevelopment/FFUUI.Core/FFUUI.Core.psm1

@@ -117,6 +117,7 @@ function Get-GeneralDefaults {
         ShareName                      = "FFUCaptureShare"
         Username                       = "ffu_user"
         Threads                        = 5
+        BitsPriority                   = 'Normal'
         MaxUSBDrives                   = 5
         BuildUSBDriveEnable            = $false
         CompactOS                      = $true