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

apps/web/src/api.ts

@@ -604,6 +604,11 @@ export interface AssignBody {
   backendIp?: string;
 }
 
+/** USB printers currently visible on the appliance (/dev/usb/lpN + sysfs model). */
+export function fetchUsbPrinters(): Promise<{ printers: { path: string; description: string | null }[] }> {
+  return apiFetch("/api/setup/usb-printers");
+}
+
 /** Save + configure the device (preconditions, push setup), then persist. */
 export function assignDevice(body: AssignBody): Promise<AssignResult> {
   return apiFetch("/api/setup/assign", { method: "POST", body: JSON.stringify(body) });