Add Opusify command to include plugins and features post-creation

[Josephat-S]

This pull request introduces a new extensible plugin system for the Opusify CLI, allowing users to add feature plugins (authentication, dark mode, analytics) to existing projects via a new add command. It implements the command handler, plugin registry, and plugin logic for each feature, supporting both Next.js and Vite project architectures where appropriate.

New Plugin System and CLI Command:

• Added a new add CLI command to Opusify, enabling users to add feature plugins (auth, dark-mode, analytics) to existing projects. The command validates the project context and plugin name, applies plugin logic, writes new files, and installs required dependencies. (index.js/src/commands/add.js) [1] [2] [3]

Feature Plugin Implementations:

Authentication Plugin (Next.js only):

• Provides NextAuth.js configuration, API route, login page, and middleware for route protection. Adds the next-auth dependency and demo credentials for local development. (src/plugins/auth.js)

Dark Mode Plugin:

• Adds a theme toggle component and theme provider for both Next.js and Vite projects, allowing users to switch between predefined themes. (src/plugins/dark-mode.js)

Analytics Plugin:

• For Next.js, adds Vercel Analytics and Speed Insights integration with a reusable provider component and setup instructions. For Vite, adds a generic analytics component and documentation for integrating Plausible or Google Analytics. Installs necessary dependencies for Next.js. (src/plugins/analytics.js)…t has been created

Closes #41