Fix screenshot functionality and add API key based ratelimiting

[connorakey]

This pull request introduces rate limiting for API requests in the backend, improves image handling for OpenAI endpoints, and adds several new dependencies to support these features. The most significant changes are the addition of per-API-key rate limiting to prevent abuse and the enhancement of image input support. Below are the top changes grouped by theme:

Rate Limiting Implementation:

• Added a new ratelimit module (backend/src/ratelimit.rs) that uses the governor, dashmap, and once_cell crates to enforce a limit of 3 requests per minute per API key. The check_rate_limit function is now called in the OpenAI endpoint to reject requests exceeding the limit. [1] [2] [3] [4]
• Updated Cargo.toml to include new dependencies: dashmap, governor, and once_cell for thread-safe rate limiting.

OpenAI Image Handling Improvements:

• Improved image input handling in both get_step_by_step_guidance and get_final_answer functions to accept either raw base64 strings or full data URLs, making the API more robust to different input formats. [1] [2]

Frontend Configuration:

• Updated the product name in the Tauri configuration file to "Maths Online App" for improved branding consistency.