Add an RZ Coin slot machine

Adds a spend-coins-for-a-random-reward mini-game reachable from the
shop. Backend mirrors the store's existing atomic/idempotent purchase
pattern (SpinAsync in CreditStore.cs) with a server-authoritative,
weighted reward roll and a rolling per-user spin-rate guard; odds are
served from GET /api/credits/spin-config so the client never hardcodes
numbers that could drift from the real payout table. The mockup's
"Streak Protector" reward was swapped for a real Exam Simulation Token
since streaks aren't currently a persisted, protectable concept.

Also fixes two latent bugs surfaced while building this:
- credits_service.gd's operation-id builder referenced a nonexistent
  AuthState.user_id (should be .username), throwing on every credit
  action.
- _post() silently no-op'd with no signal when signed out, which
  combined with a GDScript closure gotcha (lambdas capture outer
  locals by value, so reassigning inside one never reached the
  caller) meant a spin attempted while unauthenticated could hang
  forever instead of surfacing an error.

scripts/ui/main.gd

@@ -361,6 +361,11 @@ func _build_ui() -> void:
 	rz_store.main = self
 	screens_container.add_child(rz_store)
 	screens["rz_store"] = rz_store
+	var slot_machine := preload("res://scripts/ui/rz_slot_machine_screen.gd").new()
+	slot_machine.name = "RzSlotMachine"
+	slot_machine.main = self
+	screens_container.add_child(slot_machine)
+	screens["slot_machine"] = slot_machine
 
 	feedback_dialog = preload("res://scripts/ui/app_feedback_dialog.gd").new()
 	feedback_dialog.name = "AppFeedbackDialog"