feat: Admin-Toggle für mobile Zeiterfassung

Backend:
- Company.mobile_stamping_enabled (BOOLEAN DEFAULT TRUE)
- CompanyOut + CompanyUpdate: neues Feld
- Migration 0027: companies.mobile_stamping_enabled

Frontend Desktop (CompanySettingsPage):
- Abschnitt 'Mobile-Ansicht' mit Toggle-Switch
- Speichert via PATCH /companies/me

Frontend Mobile (MobileStampScreen):
- Lädt mobile_stamping_enabled aus GET /companies/me
- Deaktiviert: Hinweis-Banner statt Buttons
  ('Einstempeln nicht verfügbar – bitte Kiosk/Desktop nutzen')
- Aktiviert: normales Verhalten

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

backend/app/schemas/company.py

@@ -21,6 +21,7 @@ class CompanyOut(BaseModel):
     personnel_number_required: bool = False
     personnel_number_mode: PersonnelNumberModeT = "manual"
     personnel_number_next: int = 1
+    mobile_stamping_enabled: bool = True
 
 
 class CompanyUpdate(BaseModel):
@@ -30,6 +31,7 @@ class CompanyUpdate(BaseModel):
     personnel_number_required: bool | None = None
     personnel_number_mode: PersonnelNumberModeT | None = None
     personnel_number_next: int | None = Field(None, ge=1)
+    mobile_stamping_enabled: bool | None = None
 
 
 class DepartmentOut(BaseModel):