fix(queue): persist clear plate state across page refresh (#410)

After clicking "Clear Plate & Start Next", refreshing the page showed
the button again because the frontend determined the state purely from
the printer's FINISH/FAILED status. The backend already tracked a
plate_cleared flag in memory but never exposed it to the frontend.

- Add plate_cleared field to PrinterStatus schema and API response
- Pass plateCleared prop to PrinterQueueWidget
- Skip clear plate UI when plate is already acknowledged
- Add 2 tests for plateCleared=true behavior

frontend/src/api/client.ts

@@ -250,6 +250,8 @@ export interface PrinterStatus {
   big_fan2_speed: number | null;     // Chamber/exhaust fan
   heatbreak_fan_speed: number | null; // Hotend heatbreak fan
   firmware_version: string | null;   // Firmware version from MQTT
+  // Queue: user has acknowledged plate is cleared for next queued print
+  plate_cleared: boolean;
 }
 
 export interface PrinterCreate {