feat: 补齐 spec 遗漏 — themes/history 接口 + cover_count/coverCount/daysSinceLastAppear

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

src/routes/core-stocks.tsx

@@ -68,6 +68,7 @@ function CoreStocksPage() {
                       {d.slice(5)}
                     </th>
                   ))}
+                  <th scope="col" className="px-2 py-2 text-right font-medium" title="覆盖题材数">题材数</th>
                   <th scope="col" className="px-2 py-2 text-right font-medium">上榜</th>
                 </tr>
               </thead>
@@ -87,6 +88,9 @@ function CoreStocksPage() {
                         </td>
                       );
                     })}
+                    <td className="px-2 py-1.5 text-right tabular-nums whitespace-nowrap text-muted-foreground">
+                      {s.coverCount ?? "·"}
+                    </td>
                     <td className="px-2 py-1.5 text-right tabular-nums whitespace-nowrap">
                       <span className="inline-flex items-center gap-0.5 text-orange-500">
                         <Flame className="h-3 w-3" />