feat: add transfer UI for Assignment

Per-row employee <Select> + Transfer button alongside the existing Return
button, shown for active (returned_at null) rows. PATCHes
/api/assignments/[id] with {employeeId}, which the route already dispatches
to transferAssignment() vs returnAssignment() based on body shape.

src/app/assignments/page.tsx

@@ -88,6 +88,14 @@ export default function AssignmentsPage() {
   const [returnErrors, setReturnErrors] = useState<Record<string, string>>({});
   const [returningId, setReturningId] = useState<string | null>(null);
 
+  // Per-row transfer mini-form state, keyed by assignment id — same shape as
+  // maintenance-records/page.tsx's close mini-form state (separate maps
+  // rather than one nested-object map, so each field stays a plain string
+  // setter).
+  const [transferEmployeeId, setTransferEmployeeId] = useState<Record<string, string>>({});
+  const [transferErrors, setTransferErrors] = useState<Record<string, string>>({});
+  const [transferringId, setTransferringId] = useState<string | null>(null);
+
   async function handleSubmit(event: FormEvent<HTMLFormElement>) {
     event.preventDefault();
     setSubmitError(null);
@@ -183,6 +191,61 @@ export default function AssignmentsPage() {
     }
   }
 
+  async function handleTransfer(id: string) {
+    setTransferringId(id);
+    setTransferErrors((previous) => {
+      const next = { ...previous };
+      delete next[id];
+      return next;
+    });
+
+    const employeeId = transferEmployeeId[id] ?? "";
+
+    try {
+      // PATCH /api/assignments/[id]'s body is `{employeeId}` for a transfer
+      // — see src/app/api/assignments/[id]/route.ts's dispatch (a body
+      // carrying employeeId is a transfer, everything else is a return).
+      // employeeId is sent even if unselected (empty string), which the
+      // server rejects with a clean 400 message surfaced below, same as any
+      // other validation failure.
+      const response = await fetch(`/api/assignments/${id}`, {
+        method: "PATCH",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({ employeeId }),
+      });
+
+      let body: unknown;
+      try {
+        body = await response.json();
+      } catch {
+        body = null;
+      }
+
+      if (!response.ok) {
+        const message =
+          body !== null &&
+          typeof body === "object" &&
+          "error" in body &&
+          typeof (body as { error: unknown }).error === "string"
+            ? (body as { error: string }).error
+            : "Request failed";
+        setTransferErrors((previous) => ({ ...previous, [id]: message }));
+        return;
+      }
+
+      setTransferEmployeeId((previous) => {
+        const next = { ...previous };
+        delete next[id];
+        return next;
+      });
+      refetch();
+    } catch {
+      setTransferErrors((previous) => ({ ...previous, [id]: "Request failed" }));
+    } finally {
+      setTransferringId(null);
+    }
+  }
+
   return (
     <div className="flex flex-col gap-8 p-6">
       <h1 className="text-xl font-semibold">Assignments</h1>
@@ -220,6 +283,7 @@ export default function AssignmentsPage() {
                   <TableCell>{assignment.created_at}</TableCell>
                   <TableCell>
                     {assignment.returned_at === null ? (
+                      <div className="flex w-56 flex-col gap-2">
                         <div className="flex flex-col gap-1">
                           <Button
                             type="button"
@@ -236,6 +300,47 @@ export default function AssignmentsPage() {
                             </p>
                           ) : null}
                         </div>
+
+                        <div className="flex flex-col gap-1">
+                          <Select
+                            value={transferEmployeeId[assignment.id] ?? ""}
+                            onValueChange={(value) =>
+                              setTransferEmployeeId((previous) => ({
+                                ...previous,
+                                [assignment.id]: value,
+                              }))
+                            }
+                          >
+                            <SelectTrigger className="w-full">
+                              <SelectValue placeholder="Transfer to..." />
+                            </SelectTrigger>
+                            <SelectContent>
+                              {employees.map((employee) => (
+                                <SelectItem key={employee.id} value={employee.id}>
+                                  {employee.name}
+                                </SelectItem>
+                              ))}
+                            </SelectContent>
+                          </Select>
+                          <Button
+                            type="button"
+                            size="sm"
+                            variant="outline"
+                            disabled={
+                              transferringId === assignment.id ||
+                              !transferEmployeeId[assignment.id]
+                            }
+                            onClick={() => handleTransfer(assignment.id)}
+                          >
+                            {transferringId === assignment.id ? "Transferring..." : "Transfer"}
+                          </Button>
+                          {transferErrors[assignment.id] ? (
+                            <p className="text-xs text-destructive">
+                              {transferErrors[assignment.id]}
+                            </p>
+                          ) : null}
+                        </div>
+                      </div>
                     ) : (
                       "-"
                     )}