Add Docker Registry support for private images

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Description

Adds support for Docker registry authentication, so users can pull images from private registries.

What’s included:

1. Backend support for managing registry credentials:

• New socket handlers to add, update, and remove registries
• Secure storage of registry credentials in the settings database
• Automatic authentication before pulling images during updates

2. Authentication features:

• Support for multiple registry configurations
• Enable/disable specific registries
• Support for insecure registry connections
• Safe handling of registry tokens with sanitized messages

3. Integration with the existing stack update flow:

• Keeps full backward compatibility with public images
• This makes it easier to work with private Docker images in Dockge

Type of change

• New feature (non-breaking change which adds functionality)

Screenshots

Checklist

• My code follows the style guidelines of this project
• I ran ESLint and other linters for modified files
• I have performed a self-review of my own code and tested it
• I have commented my code, particularly in hard-to-understand areas (including JSDoc for methods)
• My changes generate no new warnings
• My code needed automated testing. I have added them (this is optional task)

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