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/NewListing.tsx

@@ -46,7 +46,7 @@ export default function NewListing() {
         </Button>
       </Box>
 
-      <Typography variant="h5" sx={{ fontWeight: 700, mb: 0.5 }}>Neues Inserat erstellen</Typography>
+      <Typography sx={{ fontWeight: 700, fontSize: '1.125rem', lineHeight: 1.3, color: '#0f172a', mb: 0.5 }}>Neues Inserat erstellen</Typography>
       <Typography variant="body2" color="text.secondary" sx={{ mb: 3 }}>
         {form.isPrefilled
           ? `Einheit ${pre.unitLabel ?? ''} aus Portfolio vorausgefüllt — Angaben prüfen und veröffentlichen.`