feat: improve billboard monitor detection and logging

inventoryManager.lua

@@ -195,12 +195,11 @@ local function main()
     end
 
     -- Billboard monitor (optional — set billboardMonitorSide in .manager_config)
-    if cfg.BILLBOARD_MONITOR ~= "" then
+    -- Billboard monitor (auto-detects any 3rd monitor, or set billboardMonitor in .manager_config)
     if display.setupBillboardMonitor() then
         log.info("INIT", "Billboard monitor: %s", display.billboardMonName)
     else
-            log.warn("INIT", "Billboard monitor not found: %s", cfg.BILLBOARD_MONITOR)
-        end
+        log.info("INIT", "No billboard monitor found (optional)")
     end
 
     -- Find wired modem for client/turtle communication