Add clear plate confirmation between queued prints

The print scheduler previously treated FINISH/FAILED as idle states
and would auto-start the next queued print before the user cleared
the build plate. Now requires explicit user confirmation via a
"Clear Plate & Start Next" button on the printer card. Uses an
in-memory plate-cleared flag — no MQTT command needed since the
scheduler's start_print overrides the printer state.

frontend/src/api/client.ts

@@ -2170,6 +2170,10 @@ export const api = {
     request<{ success: boolean; message: string }>(`/printers/${printerId}/print/resume`, {
       method: 'POST',
     }),
+  clearPlate: (printerId: number) =>
+    request<{ success: boolean; message: string }>(`/printers/${printerId}/clear-plate`, {
+      method: 'POST',
+    }),
 
   // Get current print user (for reprint tracking - Issue #206)
   getCurrentPrintUser: (printerId: number) =>