[FEATURE] Add background removal command

[Maelkiz]

🚀 Feature Request

Add a new command simply rmbg <path-to-img> to remove backgrounds from images using AI-powered background removal.

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🎯 Problem / Motivation

Background removal is a common image editing task. Adding this functionality would extend the CLI tool's capabilities and provide users with a convenient way to remove backgrounds without needing external tools or services.

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💡 Proposed Solution

Implement the simply rmbg <path-to-img> command with the following approach:

1. Use the rembg-rs crate - A Rust binding for the rembg library that provides background removal functionality
2. Machine Learning Model: The implementation will use the U2-Net model (~280 MB) for background removal
3. User Consent Flow: On the user's first use of the command:
   • Prompt the user asking if they are willing to download the ~280 MB U2-Net machine learning model
   • Only proceed with downloading and using the model if the user consents
   • Cache the model locally for subsequent uses
4. Output: Save the image with the background removed (transparent background in PNG format)

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✅ Acceptance Criteria

• simply rmbg <path-to-img> command is implemented
• The command uses the rembg-rs crate for background removal
• User is prompted on first use to consent to downloading the ~280 MB U2-Net model
• The ML model is cached locally after first download
• The command successfully removes backgrounds from images
• Output image has a transparent background (PNG format)
• Error handling for invalid image paths and unsupported formats
• Documentation is updated with usage examples

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🧠 Notes / Context (if applicable)

• rembg-rs crate: https://crates.io/crates/rembg-rs
• U2-Net model: A deep learning model trained for salient object detection, commonly used for background removal
• Consider adding a --force-download flag to allow users to re-download the model if needed
• Consider adding output path customization with an optional --output flag

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