feat(printers): AMS slot Load / Unload from the printer card (#891)

The ams_load_filament / ams_unload_filament MQTT primitives existed
  in bambu_mqtt.py but were unused — no HTTP route and no UI. Surface
  both as POST /printers/{id}/ams/load?tray_id={int} and
  POST /printers/{id}/ams/unload, gated on PRINTERS_CONTROL.

  Wire them into the existing AMS slot popover (next to "Re-read RFID")
  and add a popover wrapper on the external spool slot which had none.
  Hidden while the printer is RUNNING, mirroring the RFID re-read
  gating. Both buttons enabled when permission is granted; the printer
  no-ops gracefully if there's nothing to do (matches BambuStudio).

  Dual-extruder H2D Ext-R support is the trickier piece. The existing
  ams_load_filament(254) capture came from a single-extruder printer
  and used slot_id=254, curr/tar=-1. Captured the Ext-R command from
  BambuStudio fresh: it sends ams_id=255, slot_id=0 (the right
  extruder index, NOT a slot index), target=255, and curr/tar = the
  actual right-nozzle temp (read from state.temperatures["nozzle_2"],
  falling back to 215 °C if cold so the printer doesn't reject the
  command on a nonsensical temp). Added that as a new branch in
  ams_load_filament; the existing tray_id=254 branch is preserved
  verbatim — no risk of regression on single-external setups.

frontend/src/api/client.ts

@@ -3095,6 +3095,20 @@ export const api = {
       { method: 'POST' }
     ),
 
+  // Load filament from a tray. trayId: 0-15 for AMS (amsId*4+slotId), 254 for external spool.
+  loadAmsTray: (printerId: number, trayId: number) =>
+    request<{ success: boolean; message: string }>(
+      `/printers/${printerId}/ams/load?tray_id=${trayId}`,
+      { method: 'POST' }
+    ),
+
+  // Unload the currently loaded filament.
+  unloadAms: (printerId: number) =>
+    request<{ success: boolean; message: string }>(
+      `/printers/${printerId}/ams/unload`,
+      { method: 'POST' }
+    ),
+
   // MQTT Debug Logging
   enableMQTTLogging: (printerId: number) =>
     request<{ logging_enabled: boolean }>(`/printers/${printerId}/logging/enable`, {