chore: ignore flutter auto-generated files

These files are regenerated by flutter pub get and contain machine-
specific paths (FLUTTER_ROOT, FLUTTER_APPLICATION_PATH). They should
not be tracked — the file header itself states "do not check into
version control".

- Update .gitignore: prepend **/ to .flutter-plugins-dependencies so
  it matches in sub-packages, add Generated.xcconfig and
  flutter_export_environment.sh patterns.
- git rm --cached the 4 files that were already tracked (keeps them
  on disk for local builds but removes from repo).

.gitignore

@@ -1,6 +1,6 @@
 # Dart / Flutter workspace caches (regenerated by `flutter pub get` / `dart pub get`)
 .dart_tool/
-.flutter-plugins-dependencies
+**/.flutter-plugins-dependencies
 .packages
 .pub-cache/
 .pub/
@@ -15,6 +15,10 @@
 **/macos/Flutter/ephemeral/
 **/windows/flutter/ephemeral/
 
+# Flutter iOS build config (regenerated on pub get; contains machine-specific paths)
+**/ios/Flutter/Generated.xcconfig
+**/ios/Flutter/flutter_export_environment.sh
+
 # IDE
 .idea/
 *.iml