feat(supply): per-unit price/availability editor + mandatory fit-out cost-bearer for shell/edelrohbau

- unit.availableFrom: explicit per-unit availability date
- UnitStructurePanel: inline expandable editor (pencil) for per-unit price + availability via useUpdateUnit
- fitOutByLandlord now boolean|undefined ("not chosen"); toggle has no default for SHELL/BASIC
- NewListing: validate() blocks save until cost-bearer chosen for Rohbau/Edelrohbau
- PropertyDetailView.saveEdit: same guard for existing listings; required asterisk + hint in both forms
- newListingMapper persists explicit tenant(false) choice (was collapsed to undefined)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/pages/supply/newListingMapper.ts

@@ -46,7 +46,7 @@ export function buildCreatePropertyInput(fields: {
   const hf = {
     floor:          floor ? parseInt(floor) : undefined,
     fitOut:         (fitOut || undefined) as 'SHELL' | 'BASIC' | 'FULL' | 'PREMIUM' | undefined,
-    fitOutByLandlord: fitOutByLandlord || undefined,
+    fitOutByLandlord,
     // MAB nur relevant, wenn der Mieter ausbaut — sonst nicht speichern
     mieterausbaubeitragPerSqm: !fitOutByLandlord && mieterausbaubeitrag ? parseInt(mieterausbaubeitrag) : undefined,
     parking:        parking ? parseInt(parking) : undefined,