Initialize Harmness Browser JetBrains plugin skeleton

- Gradle + Kotlin + IntelliJ Platform SDK build
- JCEF embedded browser tool window with configurable start URL
- Settings page (Tools > Harmness Browser)
- JS<->Java bridge scaffold (window.harmnessBridge)
- Vite + React + TypeScript web scaffold for embedded page
- README: build, register, upload to Marketplace, install guide

.gitignore

@@ -0,0 +1,15 @@
+# Gradle
+.gradle/
+build/
+
+# IDE
+.idea/
+*.iml
+.vscode/
+
+# Node
+node_modules/
+web/dist/
+
+# OS
+.DS_Store