chore: update .gitignore and add .dockerignore for Dart/Flutter generated files

.gitignore

@@ -5,3 +5,7 @@ node_modules/
 package-lock.json
 
 .env.*
+
+# Dart/Flutter generated files with absolute host paths — must not be committed
+.dart_tool/
+*/.dart_tool/

flutter/.dockerignore

@@ -0,0 +1,12 @@
+# Generated by flutter pub get — contains absolute host paths
+.dart_tool/
+
+# Build output
+build/
+
+# IDE
+.idea/
+.vscode/
+
+# Dart analysis cache
+.dart_tool/dart_stagehand_template/