📸 Create screenshot assets directory for README and Chrome Web Store

[projekta2]

## Description The README has placeholders for screenshots but they are not yet added. We should create an assets/` folder and add:

• popup-screenshot.png (TabCost popup)
• audit-screenshot.png (Tab audit table)
• history-chart.png (Pro history chart)
• chrome-store-banner.png (optional)

Acceptance criteria

• Folder assets/ created in root
• At least 3 screenshots added with descriptive names
• README updated to reference actual images

Good first issue – I can provide the screenshots if you help with formatting.`

[projekta2]

I'll provide the actual screenshots (I have them locally). The task here is to:

1. Create an assets/ folder in the root of the repository
2. Add the following files (I will supply them):
   • popup-screenshot.png
   • audit-screenshot.png
   • history-chart.png
3. Update the README to replace the placeholder image links with the actual paths (assets/...)

If you'd like to help with the folder structure and README updates, leave a comment. I can then attach the images so you can add them to the PR.

Otherwise, I'll close this issue once I've added the images myself. Let me know what works for you.