Fix stale AMS slot data and empty slot button visibility (#784)

Some printers (e.g. H2D) only send {id, state} in incremental MQTT
  tray updates — no tray_type, tray_color, or other fields. When
  filament is unloaded (state changes from 11 to 10), the old tray
  data persisted indefinitely because the merge logic only updates
  fields present in the incoming message.

  Backend:
  - Detect tray state != 11 without tray_type in incremental updates
    and clear stale tray data (bambu_mqtt.py)
  - Expose tray `state` field via REST API and WebSocket broadcasts
    (printer.py schema, printers.py route, printer_manager.py)
  - Include AMS tray content in WebSocket dedup key so load/unload
    transitions trigger broadcasts (main.py)

  Frontend:
  - Add `state` to AMSTray interface (client.ts)
  - Show configure/assign buttons for state=10 (spool present, not
    loaded) but hide for state=9 (truly empty) on AMS/HT slots
  - Hide fill level bar on empty slots

  Tests:
  - 5 new tests covering state-based clearing, preservation, reload,
    and idempotency

frontend/src/api/client.ts

@@ -129,6 +129,7 @@ export interface AMSTray {
   nozzle_temp_max: number | null;  // Max nozzle temperature
   drying_temp: number | null;      // RFID-recommended drying temp
   drying_time: number | null;      // RFID-recommended drying time (hours)
+  state: number | null;            // AMS tray state: 9=empty, 10=spool present not loaded, 11=loaded
 }
 
 export interface AMSUnit {