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/MarketIntelligence.tsx

@@ -1046,8 +1046,8 @@ export default function MarketIntelligence() {
   return (
     <Box sx={{ display: 'flex', flexDirection: 'column', height: '100%', overflow: 'hidden' }}>
       {/* Page header */}
-      <Box sx={{ px: 3, py: 2, bgcolor: 'white', borderBottom: '1px solid #e2e8f0', flexShrink: 0 }}>
-        <Typography variant="h6" sx={{ fontWeight: 700, lineHeight: 1.2 }}>Marktchancen</Typography>
+      <Box sx={{ px: 3, py: 2.5, bgcolor: 'white', borderBottom: '1px solid #e8e7e4', flexShrink: 0 }}>
+        <Typography sx={{ fontWeight: 700, fontSize: '1.125rem', lineHeight: 1.3, color: '#0f172a' }}>Marktchancen</Typography>
         <Typography variant="caption" color="text.secondary">
           KI-Signale aus Web-Quellen und menschliche Netzwerk-Hinweise — mit Portfolio-Match
         </Typography>