未割り当て圃場に交互背景色を追加

行の対応が分かりにくい問題を解消するため、
偶数行/奇数行で白/グレーの交互背景色を適用。

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/app/distribution/_components/DistributionEditPage.tsx

@@ -531,9 +531,9 @@ export default function DistributionEditPage({ planId }: Props) {
               {unassignedFields.length > 0 && (
                 <div className="mt-4 border-t border-gray-200 pt-4">
                   <p className="text-xs font-medium text-gray-500 uppercase mb-2">未割り当て圃場</p>
-                  <div className="space-y-1">
-                    {unassignedFields.map(fi => (
-                      <div key={fi.id} className="flex items-center gap-2 text-sm">
+                  <div className="rounded border border-gray-200 overflow-hidden">
+                    {unassignedFields.map((fi, idx) => (
+                      <div key={fi.id} className={`flex items-center gap-2 text-sm px-3 py-1.5 ${idx % 2 === 0 ? 'bg-white' : 'bg-gray-50'}`}>
                         <span className="text-gray-800 font-medium flex-1 min-w-0 truncate" title={fi.name}>{fi.name}</span>
                         <span className="text-gray-400 text-xs w-16 shrink-0 text-right">{fi.area_tan}反</span>
                         <select