Draw active indication atop the activity bar icons when shown at the bottom (#286694)

* Initial plan

* Fix activity bar active indicator position when shown at bottom

When the activity bar is positioned at the bottom of the sidebar,
the active indicator (underline) now renders above the icons instead
of below them. This provides better visual feedback for the active item.

Changes:
- paneCompositePart.css: Separate styling for header (top) and footer (bottom)
  positions, with indicator at bottom for header and top for footer
- sidebarpart.css: Updated border color rules for footer position
- auxiliaryBarPart.css: Updated border color rules for footer position

Co-authored-by: benibenj <44439583+benibenj@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: benibenj <44439583+benibenj@users.noreply.github.com>