Setup wizard: show + override the backend push IP (multi-NIC hosts)

The backend IP baked into a push-capable device at assign time is
auto-derived by subnet-matching a local NIC. That's non-deterministic
when two NICs match the device subnet, and null when none does. Surface
it: backendIpCandidates() lists all local IPv4 NICs (on-subnet first),
GET /api/setup/backend-ips serves them, and the wizard renders an
editable Backend push IP dropdown after a successful test (pre-filled
with the auto-pick, warns when no NIC is on the device subnet). The
chosen IP overrides the auto-pick on assign and is recorded in config.

apps/web/src/SetupWizard.tsx

@@ -2,8 +2,10 @@ import { useState, useEffect } from "react";
 import {
   assignDevice,
   discoverDevices,
+  fetchBackendIps,
   fetchCatalog,
   testDevice,
+  type BackendIpCandidate,
   type Catalog,
   type CatalogEntry,
   type DeviceCategory,
@@ -102,6 +104,39 @@ function CategoryPicker({
   const [scanning, setScanning] = useState(false);
   const [scanError, setScanError] = useState<string | null>(null);
 
+  // Backend push IP: which of OUR addresses the device should call back on. We
+  // auto-pick the NIC on the device's subnet, but surface it editable here so a
+  // multi-NIC host can be corrected (the chosen IP is baked into the device on
+  // save). Only relevant for drivers that push (the field hides if no candidates).
+  const [backendIps, setBackendIps] = useState<BackendIpCandidate[] | null>(null);
+  const [backendIp, setBackendIp] = useState<string>("");
+
+  // (Re)load backend-IP candidates whenever the device host changes after a
+  // successful test (the test confirms the host is real + reachable).
+  const testedHost = tested ? String(mergedConfig().host ?? "") : "";
+  useEffect(() => {
+    if (!testedHost) {
+      setBackendIps(null);
+      return;
+    }
+    let live = true;
+    fetchBackendIps(testedHost)
+      .then(({ candidates }) => {
+        if (!live) return;
+        setBackendIps(candidates);
+        // Pre-fill with the on-subnet auto-pick (the first candidate, since the
+        // server sorts on-subnet first), unless the admin already chose one.
+        setBackendIp((cur) => cur || candidates.find((c) => c.onDeviceSubnet)?.ip || "");
+      })
+      .catch(() => {
+        if (live) setBackendIps(null);
+      });
+    return () => {
+      live = false;
+    };
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [testedHost]);
+
   async function scan() {
     if (!selected) return;
     setScanning(true);
@@ -157,7 +192,13 @@ function CategoryPicker({
     setSaving(true);
     setSaveError(null);
     try {
-      await assignDevice({ lane, category, driverId: selected.id, config: mergedConfig() });
+      await assignDevice({
+        lane,
+        category,
+        driverId: selected.id,
+        config: mergedConfig(),
+        ...(backendIp ? { backendIp } : {}),
+      });
       setSaved(true);
     } catch (e) {
       setSaveError((e as Error).message);
@@ -260,6 +301,36 @@ function CategoryPicker({
               )}
             </div>
           )}
+
+          {/* Backend push IP — only for push-capable devices (candidates present).
+              Pre-filled with the auto-pick; editable for multi-NIC hosts. */}
+          {backendIps && backendIps.length > 0 && (
+            <div style={{ margin: "0.5rem 0 0" }}>
+              <label>
+                Backend push IP{" "}
+                <select value={backendIp} onChange={(e) => setBackendIp(e.target.value)}>
+                  {!backendIps.some((c) => c.onDeviceSubnet) && (
+                    <option value="" disabled>
+                      Choose an address…
+                    </option>
+                  )}
+                  {backendIps.map((c) => (
+                    <option key={c.ip} value={c.ip}>
+                      {c.ip} ({c.iface}){c.onDeviceSubnet ? " — on device subnet" : ""}
+                    </option>
+                  ))}
+                </select>
+              </label>
+              {!backendIps.some((c) => c.onDeviceSubnet) && (
+                <span style={{ marginLeft: 8, color: "#d97706" }}>
+                  ⚠ no NIC on the device's subnet — the device may not reach the backend
+                </span>
+              )}
+              <p style={{ margin: "0.25rem 0 0", color: "#666", fontSize: "0.85em" }}>
+                The address this device will POST input events to.
+              </p>
+            </div>
+          )}
           {saveError && <p style={{ color: "crimson", margin: "0.5rem 0 0" }}>Save failed: {saveError}</p>}
           {saved && <p style={{ color: "#16a34a", margin: "0.5rem 0 0" }}>Saved and configured ✓</p>}
         </div>