fix build num

apps/edr-freight-web/backoffice/src/components/trainBuilder/BuildTrainModal.tsx

@@ -16,6 +16,7 @@ import { useEffect, useState } from "react";
 import { api } from "@/services/api";
 import type { TrainComposition } from "@/services/trainBuilder.service";
 import { useToast } from "@/hooks/use-toast";
+import { EXPORT_TRAIN_OPTIONS, importRunFor } from "@/constants/trainRuns";
 
 const parseError = (error: unknown, fallback: string) => {
   if (isAxiosError(error)) {
@@ -26,10 +27,6 @@ const parseError = (error: unknown, fallback: string) => {
   return fallback;
 };
 
-// Run-number parity carries the trade direction: odd = export, even = import.
-const isOddNumber = (value: string) => /^\d*[13579]$/.test(value.trim());
-const isEvenNumber = (value: string) => /^\d*[02468]$/.test(value.trim());
-
 /**
  * Step one of the Train Builder: pick the yard it is being assembled in and
  * couple at least two locomotives from that yard. The train code is assigned by
@@ -59,6 +56,12 @@ export default function BuildTrainModal({ opened, onClose, onBuilt }: BuildTrain
     setLocomotiveIds([]);
   }, [yardId]);
 
+  // The import run is fixed by the export run, so it tracks it rather than
+  // being entered by hand (and clears back to empty when the export is cleared).
+  useEffect(() => {
+    setImportTrainNumber(importRunFor(exportTrainNumber));
+  }, [exportTrainNumber]);
+
   useEffect(() => {
     if (!opened) {
       setExportTrainNumber("");
@@ -78,9 +81,11 @@ export default function BuildTrainModal({ opened, onClose, onBuilt }: BuildTrain
       });
       return;
     }
-    if (!isOddNumber(exportTrainNumber) || !isEvenNumber(importTrainNumber)) {
+    // Both numbers come from the fixed run pairs, so parity cannot be wrong —
+    // only "nothing picked" is reachable here.
+    if (!exportTrainNumber || !importTrainNumber) {
       toast({
-        title: "Enter both run numbers — export must be odd (e.g. 8001), import even (e.g. 8002)",
+        title: "Pick an export train number (e.g. 8001) — the import run follows it",
         variant: "destructive",
       });
       return;
@@ -133,31 +138,24 @@ export default function BuildTrainModal({ opened, onClose, onBuilt }: BuildTrain
           maxLength={100}
         />
         <Group grow>
-          <TextInput
+          <Select
             label="Export train number"
             description="Odd — Ethiopia → Djibouti runs"
             placeholder="e.g. 8001"
-            value={exportTrainNumber}
-            onChange={(e) => setExportTrainNumber(e.currentTarget.value)}
-            maxLength={20}
-            error={
-              exportTrainNumber && !isOddNumber(exportTrainNumber)
-                ? "Must be numeric and odd"
-                : undefined
-            }
+            data={EXPORT_TRAIN_OPTIONS}
+            value={exportTrainNumber || null}
+            onChange={(value) => setExportTrainNumber(value ?? "")}
+            searchable
+            clearable
           />
+          {/* Fixed by the export run — derived, never typed. */}
           <TextInput
             label="Import train number"
             description="Even — Djibouti → Ethiopia runs"
             placeholder="e.g. 8002"
             value={importTrainNumber}
-            onChange={(e) => setImportTrainNumber(e.currentTarget.value)}
-            maxLength={20}
-            error={
-              importTrainNumber && !isEvenNumber(importTrainNumber)
-                ? "Must be numeric and even"
-                : undefined
-            }
+            readOnly
+            variant="filled"
           />
         </Group>
         <Select