fix(image-to-ico): 调整下载按钮显示逻辑与预览样式

- 多个尺寸时显示 ZIP 下载主按钮，单个尺寸时显示 ICO 下载主按钮
- 优化预览尺寸文字样式与布局

src/views/image-to-ico/ImageToIcoView.vue

@@ -117,11 +117,11 @@
           </p>
         </div>
         <div class="result-actions">
-          <t-button theme="primary" @click="downloadIco">下载 ICO</t-button>
           <!-- 多个尺寸时支持把每个尺寸的单独 ICO 打包成 ZIP 一键下载 -->
-          <t-button v-if="result.singles.length > 1" variant="outline" :loading="zipping" @click="downloadAllZip">
+          <t-button v-if="result.singles.length > 1" theme="primary" :loading="zipping" @click="downloadAllZip">
             打包下载 ZIP
           </t-button>
+          <t-button v-else theme="primary" @click="downloadIco">下载 ICO</t-button>
         </div>
       </div>
 
@@ -141,7 +141,7 @@
             />
           </div>
           <div class="preview-meta">
-            <span class="preview-size">{{ p.size }}×{{ p.size }}</span>
+            <div class="preview-size">{{ p.size }}×{{ p.size }}</div>
             <div class="preview-btns">
               <t-button size="small" variant="text" theme="primary" @click="downloadPng(p)">PNG</t-button>
               <t-button size="small" variant="text" theme="primary" @click="downloadSingleIco(p.size)">ICO</t-button>
@@ -509,8 +509,9 @@ onBeforeUnmount(() => {
 }
 .preview-meta {
   display: flex;
+  flex-direction: column;
   align-items: center;
-  justify-content: space-between;
+  justify-content: center;
   margin-top: 6px;
 }
 .preview-btns {
@@ -518,7 +519,7 @@ onBeforeUnmount(() => {
   align-items: center;
 }
 .preview-size {
-  font-size: 12px;
+  font-size: 13px;
   color: var(--td-text-color-secondary);
 }
 @media (max-width: 767px) {