[explorer] make header sticky

[ukorvl]

The header

should be sticky on explorer page and on its sub-pages for transaction block and address. It means, when a user scrolls down, header should be still visible on top of the screen.

[emarc99]

Would love to give it a go!

I'll implement the sticky header functionality for the explorer page and its sub-pages (transaction block and address pages). Here's my approach:

1. I'll modify the header component to use position: sticky with top: 0 to ensure it remains visible when users scroll down the page
2. I'll add the appropriate z-index to ensure the header appears above other content
3. I'll test across different viewport sizes to ensure consistent behavior on mobile, tablet and desktop screens
4. I'll ensure the sticky behavior works correctly on all target sub-pages (explorer main page, transaction block page, and address page)
5. I'll add subtle shadow/border styling to improve visual separation when the header is in sticky mode

This enhancement will improve user experience by maintaining access to navigation and key controls while exploring lengthy content on these pages. I'm familiar with implementing sticky UI components and can complete this efficiently.

ETA: 3hrs

[emarc99]

I appreciate the opportunity.