refactor: unit-centric lease view — Einheiten & Mietverträge

Replace building-level Miet- & Mieterinformationen section with
unit-level lease display. Each unit row now shows tenant name and
lease end date directly; expand reveals full contract details
(Mietbeginn, Mietende, Laufzeit, Breakout, source system, doc link).

Floor grouping groups consecutive same-floor units under one label.
Domain model extended with Lease and Tenant types; deprecated
property-level tenant fields preserved via @deprecated for compat.

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

src/domain/lease.ts

@@ -0,0 +1,20 @@
+import type { Tenant } from './tenant'
+
+export interface Lease {
+  id: string
+  unitId: string
+  tenant: Tenant
+  rentPerSqm: number             // CHF/m²/Jahr
+  startDate: string              // ISO date
+  endDate: string                // ISO date
+  contractDurationYears?: number
+  breakoutOption?: boolean
+  breakoutOptionDate?: string
+  status: 'ACTIVE' | 'FUTURE' | 'EXPIRED'
+  sourceSystem?: 'SAP_REFX' | 'GARAIO_REM' | 'MANUAL' | string
+  sourceId?: string              // ID in source ERP
+  importedAt?: string
+  lastUpdatedAt?: string
+  contractDocumentUrl?: string
+  contractDocumentName?: string
+}