Add custom image upload feature to Background patterns

[satya-666]

Feature Request

Currently, users can upload custom decorations via the "Upload your own decoration" option in the Decorations tab. However, this functionality is not available for the Background patterns tab.

Proposed Enhancement

Add an "Upload your own background pattern" option to the Background tab, allowing users to upload custom background patterns/images similar to the existing functionality in the Decorations section.
Benefits

Increased customization: Users can create truly unique GitHub profile headers with their own background patterns

Consistency: Aligns the Background tab functionality with the Decorations tab

User flexibility: Allows users to use branded patterns, personal designs, or custom textures
Better user experience: Reduces the need to modify code directly for custom backgrounds

Current Behavior

The Background tab only offers pre-defined pattern options (squares, waves, geometric patterns, etc.)

Expected Behavior

Add an "Upload your own background pattern" button/section in the Background tab that allows users to:

Select an image file from their device
Preview how it looks as a background pattern
Apply it to their GitHub profile header

Additional Context

This feature would be particularly useful for:

Users wanting to match their brand colors/patterns
Developers wanting to use custom textures
Creating more personalized and unique profile headers

Screenshots

Labels to add: enhancement, hacktoberfest, good first issue (if you think it's suitable for new contributors)

[Gaurav-meena95]

@satya-666 can you please assign to me this