fix: parse saturation param from theme URL config

[teyepe]

Description

This PR fixes a bug where the saturation parameter in theme URL configs is ignored, causing themes to always load with 100% saturation regardless of the URL parameter value.

Problem

When users share Leonardo theme URLs with a specific saturation value in the config (e.g., "saturation": 20), the web UI ignores this parameter and defaults to 100% saturation. This breaks the ability to share complete theme configurations via URL.

Fix

Added saturation parameter parsing that:

1. Reads config.saturation from the URL parameter
2. Defaults to 100 if not specified (maintaining backward compatibility)
3. Updates the saturation slider UI element
4. Updates the saturation display label
5. Sets _theme.saturation property

The implementation follows the exact same pattern used for the existing contrast parameter (lines 112-116), ensuring consistency with the codebase. The fix is minimal and improves theme sharing.

• [ x] I have read the CONTRIBUTING document.

• [ x] This pull request is ready to merge.

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Latest commit: d41adbf436e80c9251b32351824b6bf9849125c8

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