Fix sticky folder background transparency in file tree

[0xDAEF0F]

Summary

• Add depth-based z-index to sticky folders so parent folders layer above children
• Fix CSS selector specificity for sticky folder background colors
• Only apply bg-transparent to file items, not directories

Known Issues

There is still some minor content bleeding through sticky headers that needs further investigation.

Test plan

• Open a deeply nested folder structure in the file explorer
• Scroll through the file tree and verify sticky folder headers hide content beneath them
• Verify parent folders appear above child folders when multiple are sticky

[mehmetozguldev]

LGTM