How do I store the Stark in winter, and can I ride in freezing weather?

Per the manual

• store at 0…+30 °C, ideally around +10 °C, dry and frost-free
• battery charge 30–50%; before storage run it down to 30% and charge to 50% — this balances the cells
• check the charge every two weeks; if it drops below 30%, top up to 50%; never let it run flat
• don't charge below +4 °C
• recommended operating range is 0…+25 °C

From the chat: a fully switched-off bike kept warm loses about 5% over winter. You can ride below zero, but with a warm battery from a warm garage and without charging in the cold; range is lower in the cold.