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/server/src/net.ts

@@ -28,3 +28,42 @@ export function backendIpForDevice(deviceHost: string): string | null {
 export function backendPort(): number {
   return Number(process.env.PORT ?? 3000);
 }
+
+export interface BackendIpCandidate {
+  ip: string;
+  iface: string;
+  /** True if this interface's subnet contains the device IP (the likely one). */
+  onDeviceSubnet: boolean;
+}
+
+/**
+ * List local IPv4 addresses the device could call back on, with the ones on the
+ * device's own subnet flagged + sorted first. Lets the admin see/override the
+ * auto-pick (important on multi-NIC hosts). BACKEND_HOST_IP, if set, is the only
+ * candidate (the deterministic override).
+ */
+export function backendIpCandidates(deviceHost: string): BackendIpCandidate[] {
+  if (process.env.BACKEND_HOST_IP) {
+    return [{ ip: process.env.BACKEND_HOST_IP, iface: "BACKEND_HOST_IP", onDeviceSubnet: true }];
+  }
+
+  const dev = deviceHost.split(".").map(Number);
+  const validDev = dev.length === 4 && !dev.some((o) => Number.isNaN(o));
+  const out: BackendIpCandidate[] = [];
+
+  for (const [iface, ifaces] of Object.entries(networkInterfaces())) {
+    for (const i of ifaces ?? []) {
+      if (i.family !== "IPv4" || i.internal) continue;
+      const addr = i.address.split(".").map(Number);
+      const mask = i.netmask.split(".").map(Number);
+      const onDeviceSubnet =
+        validDev &&
+        addr.length === 4 &&
+        mask.length === 4 &&
+        dev.every((o, k) => (o & mask[k]!) === (addr[k]! & mask[k]!));
+      out.push({ ip: i.address, iface, onDeviceSubnet });
+    }
+  }
+  // On-subnet candidates first.
+  return out.sort((a, b) => Number(b.onDeviceSubnet) - Number(a.onDeviceSubnet));
+}