fix: MyListings — sticky header bar matches standard page layout

Restructured from scrollable content-box to flex-column with a
fixed header bar (borderBottom #e8e7e4, py:2.5) and a scrollable
content area below — identical pattern to all other pages.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/pages/supply/MyListings.tsx

@@ -339,8 +339,9 @@ export default function MyListings() {
   }
 
   return (
-    <Box sx={{ maxWidth: 1040, mx: 'auto', px: 3, py: 4 }}>
+    <Box sx={{ display: 'flex', flexDirection: 'column', height: '100%', overflow: 'hidden' }}>
-      <Box sx={{ display: 'flex', alignItems: 'center', justifyContent: 'space-between', mb: 3 }}>
+      {/* Header */}
+      <Box sx={{ bgcolor: 'white', borderBottom: '1px solid #e8e7e4', px: 3, py: 2.5, flexShrink: 0, display: 'flex', alignItems: 'center', justifyContent: 'space-between' }}>
         <Box>
           <Typography sx={{ fontWeight: 700, fontSize: '1.125rem', lineHeight: 1.3, color: '#0f172a' }}>Inserate</Typography>
           <Typography variant="body2" color="text.secondary">
@@ -357,6 +358,8 @@ export default function MyListings() {
         </Button>
       </Box>
 
+      <Box sx={{ flex: 1, overflowY: 'auto', px: 3, py: 3, maxWidth: 1040, width: '100%', mx: 'auto', boxSizing: 'border-box' }}>
+
       {actionError && (
         <Alert severity="error" sx={{ mb: 2 }} onClose={() => setActionError(null)}>
           {actionError}
@@ -415,6 +418,8 @@ export default function MyListings() {
         </Box>
       )}
 
+      </Box>{/* end scroll container */}
+
       {/* Detail / edit drawer */}
       <Drawer
         anchor="right"