feat(setup): USB printer discovery — pick a real /dev/usb device

The kernel numbers usblp nodes by plug/boot order (park-buzi's printer
is lp1); the wizard hardcoded lp0 in labels/default and the admin had to
shell in and `ls /dev/usb`. Now:

- GET /api/setup/usb-printers enumerates /dev/usb/lpN (visible via the
  compose bind-mount) and enriches each with the printer's self-reported
  make/model from sysfs ieee1284_id (readable through Docker's ro /sys).
- The wizard's devicePath becomes a SELECT of printers actually present
  ("/dev/usb/lp1 — Xprinter XP-K200L"): a fresh form preselects the
  first real device; a saved-but-unplugged path stays selectable,
  flagged "saved — not present now"; zero found falls back to free text
  + a check-the-cable hint.
- Transport option label no longer hardcodes lp0.

Wiki: printer-usb-transport marked HARDWARE-VERIFIED (lab 2026-07-07:
full slip + feed + cut over USB — parity with TCP).

Claude-Session: https://claude.ai/code/session_01Xcm6ikLgGoCxxHrxtjkk5V

packages/devices/src/drivers/printer-escpos.ts

@@ -775,7 +775,7 @@ export const transportField: ConfigField = {
   default: "tcp-ip",
   options: [
     { value: "tcp-ip", label: "Network (raw TCP)" },
-    { value: "usb", label: "USB (local /dev/usb/lp0)" },
+    { value: "usb", label: "USB (local printer)" },
   ],
   help: "USB drives a printer plugged into the appliance (usblp); Network drives one on the isolated device VLAN.",
 };
@@ -787,5 +787,5 @@ export const devicePathField: ConfigField = {
   type: "string",
   required: false,
   default: "/dev/usb/lp0",
-  help: "Character device for a USB printer (usblp), e.g. /dev/usb/lp0. Only used when Connection is USB.",
+  help: "usblp character device (/dev/usb/lpN). The setup UI lists the printers actually present; the kernel numbers them (lp0, lp1, …) by plug/boot order. Only used when Connection is USB.",
 };