Organize gitignore and ignore local editor caches

Merge PR #37: organize .gitignore and ignore local editor/build cache files.

Author: hawkff

.gitignore

@@ -1,20 +1,34 @@
-*.iml
-.gradle
-.idea
-.vscode
+# OS / filesystem
 .DS_Store
+._*
+__MACOSX/
+
+# IDE / editor
+*.iml
+.idea/
+.vscode/
+.cursor/
+
+# Gradle / Kotlin build state
+.gradle/
+.kotlin/
 build/
-/captures
-.externalNativeBuild
-.cxx
+
+# Android / native build outputs
+/captures/
+.externalNativeBuild/
+.cxx/
+jniLibs/
+/library/libcore_build/
+/libcore/.build/
+/.masterdnsvpn-build/
+
+# Local config / generated app artifacts
 local.properties
 /app/libs/
 /app/src/main/assets/sing-box
 /service_account_credentials.json
-jniLibs/
-/library/libcore_build/
-.idea/deploymentTargetDropDown.xml
-/nkmr
+/nkmr/
 
-# submodules
-/external
+# Submodules / external sources
+/external/