feat: 横向对比增加单买成本列（第一列）

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/views/KitExport.vue

@@ -77,12 +77,14 @@
           <thead>
             <tr>
               <th class="th-name">配方名</th>
+              <th class="th-kit">单买</th>
               <th v-for="ka in kitAnalysis" :key="ka.id" class="th-kit">{{ ka.name }}</th>
             </tr>
           </thead>
           <tbody>
             <tr v-for="row in crossComparison" :key="row.id">
               <td class="td-name">{{ row.name }}</td>
+              <td class="td-cost original">{{ fmtPrice(row.originalCost) }}</td>
               <td v-for="ka in kitAnalysis" :key="ka.id" :class="row.costs[ka.id] != null ? 'td-kit-available' : 'td-kit-na'">
                 <template v-if="row.costs[ka.id] != null">{{ fmtPrice(row.costs[ka.id]) }}</template>
                 <template v-else><span class="na">—</span></template>