Add stagger to Print dialog, add plate-clear setting (#752)

Stagger option now available when printing directly to multiple printers,
  not just in queue mode. Prints are automatically queued with staggered
  start times using group size/interval from Settings. New "Require
  plate-clear confirmation" setting lets farm users disable per-printer
  plate confirmations so queued prints start automatically on finished
  printers.

  Also fixes settings API type parsing for require_plate_clear (boolean),
  stagger_group_size and stagger_interval_minutes (integer) — without this,
  saved values returned as strings would cause the settings toggle to
  always show enabled and trigger a permanent save loop.

frontend/src/api/client.ts

@@ -889,6 +889,8 @@ export interface AppSettings {
   // Staggered batch start defaults
   stagger_group_size: number;
   stagger_interval_minutes: number;
+  // Plate-clear confirmation
+  require_plate_clear: boolean;
   // Default sidebar order (admin-set for all users)
   default_sidebar_order: string;
 }