refactor: standardize page title typography across all workspaces

All page-level headers now use fontSize:1.125rem, fontWeight:700,
color:#0f172a, container py:2.5, borderBottom:#e8e7e4 — matching
the standard established in PageHeader. Removes variant="h5"/h6"
inconsistencies across Supply, Demand, and shared components.

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

src/pages/supply/Anfragencenter.tsx

@@ -9,8 +9,8 @@ export default function Anfragencenter() {
   const [tab, setTab] = useState(initialTab)
   return (
     <Box sx={{ display: 'flex', flexDirection: 'column', height: '100%', overflow: 'hidden' }}>
-      <Box sx={{ borderBottom: '1px solid #e2e8f0', px: 3, bgcolor: 'white', flexShrink: 0 }}>
-        <Typography variant="h5" sx={{ fontWeight: 700, pt: 2.5, pb: 1 }}>
+      <Box sx={{ borderBottom: '1px solid #e8e7e4', px: 3, bgcolor: 'white', flexShrink: 0 }}>
+        <Typography sx={{ fontWeight: 700, fontSize: '1.125rem', lineHeight: 1.3, color: '#0f172a', pt: 2.5, pb: 1, display: 'block' }}>
           Anfragencenter
         </Typography>
         <Tabs