Auto-detect subnet for printer discovery

The Add Printer dialog previously required users to manually enter their
network subnet for scanning (defaulting to 192.168.1.0/24). Now the
backend detects available network interfaces and returns their subnets
via the /discovery/info endpoint. The frontend auto-selects the first
detected subnet and shows a dropdown when multiple subnets are available,
falling back to a text input if none are detected.

frontend/src/api/client.ts

@@ -3934,6 +3934,7 @@ export interface DiscoveryInfo {
   is_docker: boolean;
   ssdp_running: boolean;
   scan_running: boolean;
+  subnets: string[];
 }
 
 export interface SubnetScanStatus {