feat: ai samples implenting text generation/chat samples

[sedanah-m]

Implement text-generation and chat samples.
Add decoupled logic so that user can run script to start either sample.
Updates to the README file and adds AppCheck debug intialization.

[gemini-code-assist]

Code Review

This pull request introduces a modular React and Vite-based application showcasing Firebase AI capabilities, including text generation and multi-turn chat. Feedback on the changes highlights several critical issues: an invalid model name ('gemini-3.5-flash') is used across multiple service files, which will cause runtime failures; the Chat view lacks error handling during initialization, potentially leading to application crashes; Firebase configuration values are hardcoded instead of using environment variables; and a deprecated @types/react-router-dom dependency is present, risking type conflicts with React Router v7.

[hsubox76]

Apps look good so far.

Can you add instructions in the README.md about how to install and run the samples, and the different options? Cover

1. how to put your project config in
2. npm install
3. how to run full app or individual samples
4. where to go in the browser
5. Can cut and paste code out of service.ts file for platform agnostic use

Also, add App Check debug initialization as we discussed (I guess just in firebaseAIService.ts).

[rafikhan]

Looked this over with Christina. The general structure of the changes look good. Other items still need work but we'll handle those in separate PRs