refactor: restructure repository with separate backend and frontend directories

- Move Java backend to backend/ directory
- Create frontend/ directory for TypeScript TUI and future WebUI
- Update .gitignore for Node.js and worktrees
- Update README.md with new repository structure
- Copy documentation to backend/

.gitignore

@@ -50,3 +50,16 @@ coverage/
 .mvn/
 mvnw
 mvnw.cmd
+
+# Node.js / Frontend
+frontend/node_modules/
+frontend/dist/
+frontend/build/
+frontend/.pnpm-store/
+frontend/**/.turbo/
+frontend/**/.next/
+*.tsbuildinfo
+.pnpm-debug.log
+
+# Git worktrees
+.worktrees/