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>

lefthook.yml

@@ -0,0 +1,25 @@
+pre-commit:
+  parallel: true
+  commands:
+    gitleaks:
+      run: gitleaks protect --staged --no-banner --redact
+    format-frontend:
+      glob: "frontend/src/**/*.{ts,tsx}"
+      run: cd frontend && npx biome format --write src/
+      stage_fixed: true
+    lint-frontend:
+      glob: "frontend/src/**/*.{ts,tsx}"
+      run: cd frontend && npx biome check src/
+    format-backend:
+      glob: "backend/src/**/*.java"
+      run: ./backend/mvnw -f backend/pom.xml spotless:apply -q
+      stage_fixed: true
+    lint-backend:
+      glob: "backend/src/**/*.java"
+      run: ./backend/mvnw -f backend/pom.xml spotless:check -q
+pre-push:
+  commands:
+    test-frontend:
+      run: cd frontend && npx vitest run
+    test-backend:
+      run: ./backend/mvnw -f backend/pom.xml test -q