chore(mobile): untrack machine-specific flutter generated files

These files (local.properties, Generated.xcconfig,
flutter_export_environment.sh, ios/Flutter/ephemeral) embed absolute
SDK paths and are regenerated per machine on every flutter command.
Their headers state "do not check into version control". Add them to
.gitignore and remove from the index to stop spurious diffs.

Co-Authored-By: First Fluke <our.first.fluke@gmail.com>

Author: gracefullight

.gitignore

@@ -58,6 +58,11 @@ apps/mobile/build/
 apps/mobile/.flutter-plugins
 apps/mobile/.flutter-plugins-dependencies
 apps/mobile/.packages
+# Flutter machine-specific generated files (do not check into version control)
+apps/mobile/android/local.properties
+apps/mobile/ios/Flutter/Generated.xcconfig
+apps/mobile/ios/Flutter/flutter_export_environment.sh
+apps/mobile/ios/Flutter/ephemeral/
 
 # Misc
 *.bak