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/layout/AppShell.tsx

@@ -39,6 +39,7 @@ import {
   Menu,
   Kanban,
   BellRing,
+  Plus,
 } from 'lucide-react'
 import type { LucideIcon } from 'lucide-react'
 import { OrganizationContextBadge } from './OrganizationContextBadge'
@@ -88,6 +89,7 @@ const WORKSPACE_CONFIG: Record<WorkspaceType, WorkspaceConfig> = {
       { path: '/supply/anfragen', label: 'Anfragencenter', icon: MessageSquare },
       { path: '/supply/data-quality', label: 'Datenpflege', icon: CheckSquare },
       { path: '/supply/market-intelligence', label: 'Markt Intelligence', icon: Radar },
+      { path: '/supply/new-listing', label: 'Neues Inserat', icon: Plus },
     ],
   },
   [WorkspaceType.DEMAND]: {