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/domain/unit.ts

@@ -17,6 +17,7 @@ export interface PropertyUnit {
   areaSqm: number
   available: boolean
   rentPricePerSqm?: number  // annual CHF/m²; falls back to property.rentPricePerSqm
+  availableFrom?: string    // explicit availability date for a free unit (ISO); else derived from leases
   // Unit-level fit-out override — falls back to property.hardFacts.fitOut when unset.
   // Relevant for pre-market, where units of one property are matched individually.
   fitOut?: 'SHELL' | 'BASIC' | 'FULL' | 'PREMIUM'