Hide the Offline notice when you prefer to cook offline

Shipped in Gratin 0.5.3. Use the × beside Offline to hide the notice on this device. To restore it, open Settings > Preferences and enable Show offline notice. Your choice is remembered across launches. Syncing and connection diagnostics continue normally. Desktop updates and the mobile update bundle are published. Android build 503 has been promoted to closed testing. Apple builds have uploaded; store processing is separate. Prioritised by the owner after the 21 September tester follow-up: the tester often uses flight mode, the notice disappears when Wi-Fi reconnects, and URL import works online. This is a display preference, separate from the previous connection-diagnostics work in FB-303395.