fix: display stuck on splash for UART boards (disconnected flag)

The display task started with disconnected=true, waiting for a
DisplayCommand::On that the USB task sends on DTR connect. The UART
task has no DTR, so the display never left the "disconnected" state
and ignored all radio status updates. Start connected instead — the
USB task will send Reset on actual disconnect.

firmware/src/display.rs

@@ -42,7 +42,7 @@ impl DisplayState {
             current_slot_rssi: NO_SIGNAL,
             current_slot_tx: false,
             display_on: true,
-            disconnected: true,
+            disconnected: false,
             last_status: RadioStatus::default(),
         }
     }