Add settings support for color input preferences

[sri443]

Objective

• Fixes The docs for FeathersColorInput claim that user preferences are saved, but this is not actually implemented #26067.
• Let users save their color input mode and recently used colors so their preferences are still there when they reopen the app.

Solution

• This PR adds bevy-settings and connects ColorInputSettings to Bevy's settings system, so apps can save and restore the selected mode and recent colors. To make this work, apps need to add SettingsPlugin and trigger a save.
• This also makes bevy-settings a direct dependency of bevy_feathers. If there's a preferred way to handle this dependency, I'm happy to adjust.
• This replaces Fix color input docs #26071.

Testing

• I ran the formatting, compilation, diff, and documentation checks:

  cargo fmt --all -- --check
  cargo check -p bevy_feathers
  git diff --check
  cargo doc -p bevy_feathers --no-deps

• I also tested this manually in a standalone app on Windows. I changed the color input mode to RGPlane, triggered a save, and verified that the settings file contained the selected mode and recent colors. After restarting the app, I confirmed that the saved mode was restored.
• No automated test added. Testing on other platforms would be welcome.

AI Disclosure

• I used AI assistance to navigate the codebase, identify the relevant files, and understand how bevy_settings works so I could integrate it with ColorInputSettings. AI also assisted with reviewing the code, while I manually implemented the changes and tested saving and loading the settings.