Add Flutter platform stubs for iOS, web, Linux, macOS, Windows

Auto-generated by flutter create; not currently targeted but kept for
future portability.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

macos/.gitignore

@@ -0,0 +1,7 @@
+# Flutter-related
+**/Flutter/ephemeral/
+**/Pods/
+
+# Xcode-related
+**/dgph
+**/xcuserdata/