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.

frontend/public/file.svg

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