feat: Inserat erstellen — direct listing flow for units and standalone

- New /supply/new-listing page (Path A: pre-filled from portfolio unit,
  Path B: standalone blank form); creates EXTERNAL_MARKET + sourceType DIRECT
- "+ Inserat" button on available units in PropertyDetailView navigates
  with prefilled address/area/rent
- Nav entry "Neues Inserat" added to supply sidebar
- SourceProvenancePanel hidden for DIRECT source listings (no external source)

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

src/components/supply/PropertyDetailView.tsx

@@ -106,6 +106,7 @@ function MatchPill({ m }: { m: UnitNeedMatch }) {
 }
 
 function UnitStructurePanel({ p }: { p: Property }) {
+  const navigate = useNavigate()
   const [selectedIds, setSelectedIds] = useState<Set<string>>(new Set())
   const [expandedUnit, setExpandedUnit] = useState<string | null>(null)
 
@@ -204,6 +205,28 @@ function UnitStructurePanel({ p }: { p: Property }) {
                       {u.isFlexible && (
                         <Chip label="Teilfläche möglich" size="small" sx={{ height: 16, fontSize: '0.58rem', bgcolor: '#eff6ff', color: '#1d4ed8', border: '1px solid #bfdbfe' }} />
                       )}
+                      <Button
+                        size="small"
+                        variant="text"
+                        sx={{ height: 16, fontSize: '0.58rem', p: 0, minWidth: 0, color: '#7c3aed', textTransform: 'none', lineHeight: 1 }}
+                        onClick={() => navigate('/supply/new-listing', {
+                          state: {
+                            prefill: {
+                              assetType: p.assetType,
+                              street: p.address?.street,
+                              houseNumber: p.address?.houseNumber,
+                              postalCode: p.address?.postalCode,
+                              city: p.address?.city,
+                              areaSqm: u.offeredSqm ?? u.areaSqm,
+                              rentPricePerSqm: u.rentPricePerSqm ?? p.rentPricePerSqm,
+                              unitLabel: u.unitLabel,
+                              propertyId: p.id,
+                            },
+                          },
+                        })}
+                      >
+                        + Inserat
+                      </Button>
                     </>
                   ) : (
                     <Typography variant="caption" sx={{ color: '#64748b' }} noWrap>{u.currentTenant ?? 'Vermietet'}</Typography>