scaffold: React 19 + Spring Boot 3.4 + PostgreSQL project

Sets up toolchain pinning, format/lint (Biome + Spotless), lefthook
pre-commit hooks, Docker Compose for Postgres, and trivial passing
tests for both stacks.

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

.gitignore

@@ -0,0 +1,27 @@
+# Java / Maven
+target/
+*.class
+*.jar
+*.war
+*.ear
+hs_err_pid*
+
+# Spring Boot
+*.log
+
+# Node
+frontend/node_modules/
+frontend/dist/
+frontend/.vite/
+
+# IDE
+.idea/
+*.iml
+.vscode/
+
+# macOS
+.DS_Store
+
+# Environment — never commit real credentials
+.env
+!.env.example