Refactor backend to Gin version

[Darrin-Lin]

This pull request migrates the project from a Node.js/TypeScript/Express backend to a Go backend using the Gin framework. It removes all Node.js/TypeScript-specific configuration, code, and tooling, replacing them with Go equivalents for dependency management, code quality, CI/CD, and documentation. The README and workflow files are rewritten to reflect the new Go-based stack and development practices.

Migration to Go backend:

• All Node.js, TypeScript, and Express-related files and configurations are removed, including package.json, eslint.config.js, and the main application code in src/app.ts. [1] [2] [3]
• Go module initialization is added via go.mod, specifying dependencies for Gin, Firebase, and supporting libraries.
• The README is completely rewritten to document the new Go-based backend, its structure, setup, development, and deployment instructions. [1] [2] [3]

CI/CD and workflow changes:

• GitHub Actions workflows (ci.yml and test.yml) are updated to use Go instead of Node.js, including dependency caching, formatting, vetting, and building/testing with Go tools. Node.js and pnpm steps are removed. [1] [2]

Code quality and pre-commit hooks:

• Husky pre-commit hook now runs Go formatting, vetting, and dependency tidy commands instead of lint-staged with ESLint/Prettier.
• lint-staged configuration is updated to format only Go files.

Environment and Firebase setup:

• .env.example is simplified for Go, and a new firebase.json is added for Firestore emulator configuration. [1] [2]

Summary of most important changes:

1. Migration to Go backend

• Removed all Node.js/TypeScript/Express code and configuration, including package.json, eslint.config.js, and src/app.ts. [1] [2] [3]
• Added Go module (go.mod) with dependencies for Gin, Firebase, and related libraries.
• Rewrote README.md to document Go stack, project structure, and development workflow. [1] [2] [3]

2. CI/CD and workflow updates

• Updated GitHub Actions workflows to use Go for building, formatting, vetting, and testing, removing Node.js and pnpm steps. [1] [2]

3. Code quality and pre-commit

• Changed Husky pre-commit hook to run Go code quality checks (go fmt, go vet, go mod tidy).
• Updated lint-staged to format Go files only.

4. Environment and Firebase configuration

• Simplified .env.example for Go and added firebase.json for Firestore emulator. [1] [2]

[Darrin-Lin]

Now have prd/test/dev mode can select in APP_ENV in .env, only test mode will open test api.
And also have option of use firestore emulator or not.
Credential file change to env and put in credentials dir.

[NaoCoding]

Try to resolve the conversation once you completed the changes.
So I can easily know the status of your PRs.