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/App.tsx

@@ -27,6 +27,7 @@ const FutureAvailability = lazy(() => import('./pages/supply/FutureAvailability'
 const DataQuality = lazy(() => import('./pages/supply/DataQuality'))
 const ReminderManager = lazy(() => import('./pages/supply/ReminderManager'))
 const MarketLeads = lazy(() => import('./pages/supply/MarketLeads'))
+const NewListing = lazy(() => import('./pages/supply/NewListing'))
 
 const AISearch = lazy(() => import('./pages/demand/AISearch'))
 const Results = lazy(() => import('./pages/demand/Results'))
@@ -62,6 +63,7 @@ function App() {
                 <Route path="/supply/reminder-manager" element={<ReminderManager />} />
                 <Route path="/supply/market-leads" element={<MarketLeads />} />
                 <Route path="/supply/market-intelligence" element={<MarketIntelligence />} />
+                <Route path="/supply/new-listing" element={<NewListing />} />
               </Route>
 
               {/* Demand Workspace */}