Initialize CMS application scaffold

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

ui/changelog.md

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+# Prompt and implementation changelog
+
+## 2026-09-19
+
+### Prompt
+
+Create a CMS React application with a Go backend, initialize a Git repository, and
+prepare the security foundation first. Add an extensible external identity-provider
+login beginning with Google, a protected test page, and an admin login at
+`/admin/login` whose link is never exposed in user-facing pages. Use Goose for
+migrations and GORM for persistence. Structure the backend into web handlers,
+web-to-domain mapping, domain-only services, database models, repositories, and
+domain/database mappers. Prefix interfaces with `I`. Use Go 1.27.1 and the latest
+React. Keep this changelog in `ui/changelog.md`.
+
+### Tasks completed
+
+- Initialized a local Git repository on the `main` branch; GitHub remote
+  configuration is intentionally left for later.
+- Added a Go 1.27.1 backend module with configuration via environment variables.
+- Added layered backend packages for domain, web handlers and mapping, services,
+  repositories, persistence models, and persistence mapping.
+- Added GORM with SQLite as the local database driver and Goose migrations for
+  users, administrators, and sessions.
+- Added an OAuth provider abstraction and Google OAuth authorization-code flow with
+  state validation, userinfo lookup, and an HTTP-only session cookie.
+- Added health, session, Google auth, protected test, and hidden admin login routes.
+- Added a React/Vite frontend with a public Google sign-in page, protected `/test`
+  page, and `/admin/login` route without a visible navigation link.
+- Added environment examples, ignore rules, README documentation, and backend
+  unit tests.
+- Verified Go tests and prepared frontend typecheck/build scripts.