Fix margin and background processing order

[devin-ai-integration]

Fix margin and background processing order

Issue Description

When both margin and background are specified in a variant configuration, the margin areas remain transparent instead of using the specified background color. This happens because the margin is applied before the background in the processing pipeline.

Changes Made

1. Reordered operations in processImage method

• Moved background application to before margin application
• This ensures the background color is set before margins are added

2. Updated applyMargin method signature

• Added optional background parameter to accept background color
• Use provided background or default to transparent if none is provided
• Replace hardcoded transparent background in extend() call with the background parameter

3. Applied same changes to ICO generation path

• Updated generateMultiSizeIcoFile method to follow same pattern
• Apply background before margin and pass background color to margin application

Implementation Details

• Background color parsing uses existing parseBackgroundColor utility
• When no background color is specified, maintains current behavior of transparent margins
• Changes applied consistently in both main processing path and ICO generation path

Testing

• All existing tests pass (73/73)
• Lint checks pass
• No breaking changes to existing functionality

Fixes MAN8-4791

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Link to Devin run: https://app.devin.ai/sessions/3b8769218ed24ce79679a8da9b8b7a16
Requested by: Louis Mandelstam (louis@man8.com)

[devin-ai-integration]

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