Add initial backend, CLI, and frontend scaffold

Bootstrap repository with multi-component scaffolding: a Spring Boot backend (Maven wrapper, pom.xml, BackendApplication, test, resources, .gitignore/.gitattributes), a Go CLI app using Cobra (root + subcommands: diff, log, login, pull, push, set, unset), prebuilt CLI binaries and a build script, and a Next.js frontend (configs, package.json, styles, pages, and static assets). This commit provides the skeleton for development across backend, CLI, and frontend components.

backend/.mvn/wrapper/maven-wrapper.properties

@@ -0,0 +1,3 @@
+wrapperVersion=3.3.4
+distributionType=only-script
+distributionUrl=https://repo.maven.apache.org/maven2/org/apache/maven/apache-maven/3.9.12/apache-maven-3.9.12-bin.zip