Follows the same pattern as AppIcon-Debug (orange DEV banner) but with
a purple banner and "NIGHTLY" text. The nightly CI workflow now passes
ASSETCATALOG_COMPILER_APPICON_NAME=AppIcon-Nightly to xcodebuild so
the nightly app gets its own distinct icon.
Includes scripts/generate_nightly_icon.py for regenerating the icons
from the production AppIcon source files.
