fix: standardize subtitle typography in all page headers

All page header subtitles now use variant="body2" color="text.secondary"
(0.875rem gray). Fixed caption→body2 in DataQuality and MarketIntelligence,
and aligned DashboardHeader title/subtitle to the global standard.

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

src/pages/supply/MarketIntelligence.tsx

@@ -1048,7 +1048,7 @@ export default function MarketIntelligence() {
       {/* Page header */}
       <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">
+        <Typography variant="body2" color="text.secondary">
           KI-Signale aus Web-Quellen und menschliche Netzwerk-Hinweise — mit Portfolio-Match
         </Typography>
       </Box>