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/demand/AISearch.tsx

@@ -208,8 +208,8 @@ export default function AISearch() {
   return (
     <Box sx={{ display: 'flex', flexDirection: 'column', height: '100%', overflow: 'hidden' }}>
       {/* Header */}
-      <Box sx={{ bgcolor: 'white', borderBottom: '1px solid #e2e8f0', px: 3, py: 2, flexShrink: 0 }}>
-        <Typography variant="h5" sx={{ fontWeight: 700 }}>Flächensuche</Typography>
+      <Box sx={{ bgcolor: 'white', borderBottom: '1px solid #e8e7e4', px: 3, py: 2.5, flexShrink: 0 }}>
+        <Typography sx={{ fontWeight: 700, fontSize: '1.125rem', lineHeight: 1.3, color: '#0f172a' }}>Flächensuche</Typography>
         <Typography variant="body2" color="text.secondary">
           Sprechen, schreiben oder Felder ausfüllen — dann sofort suchen oder als Suchprofil speichern
         </Typography>