Fix sidebar covering header on scroll in Feed Detail Map page

The StyledMapControlPanel had zIndex: 10000 (needed for mobile full-screen
overlay) but this was not reset for desktop (md) breakpoint. On desktop the
panel has position: relative, so the high z-index created a stacking context
that rendered above the fixed AppBar header (z-index 1100) when scrolling.

Fix: add zIndex: 'auto' to the md breakpoint to restore normal document
stacking order on desktop.

Closes #57

Co-authored-by: Alessandro100 <18631060+Alessandro100@users.noreply.github.com>

Author: Copilot

src/app/screens/Feed/Map.styles.ts

@@ -29,6 +29,7 @@ export const StyledMapControlPanel = styled(Box, {
     top: 'unset',
     paddingTop: 0,
     margin: theme.spacing(2),
+    zIndex: 'auto',
   },
 }));