fix: Logo左下角+日期靠右，手机字号自适应，精油名不换行

配方卡片:
- Logo: 左下角靠左对齐(flex)，opacity:0.5
- 制作日期: 靠右对齐（Logo和日期同一行，space-between）
- 精油名: white-space:nowrap + text-overflow:ellipsis，不换行
- 配方名: max-width留QR空间，不换行
- 手机端(@media max-width:420px): 整体字号缩小，padding缩小

预览图:
- 同样Logo左下+日期靠右布局
- 去掉了底部居中Logo水印

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

frontend/src/views/MyDiary.vue

@@ -171,8 +171,7 @@
           <div class="card-preview-mini">
             <!-- Background overlay -->
             <div v-if="brandBg" style="position:absolute;inset:0;background-size:cover;background-position:center;opacity:0.12;pointer-events:none" :style="{ backgroundImage: 'url(' + brandBg + ')' }"></div>
-            <!-- Logo watermark: bottom center -->
-            <img v-if="brandLogo" :src="brandLogo" style="position:absolute;bottom:34px;left:50%;transform:translateX(-50%);height:30px;opacity:0.2;pointer-events:none;z-index:1" />
+            <!-- Logo: shown in bottom row, not as watermark -->
             <!-- QR: top-right -->
             <div v-if="brandQrImage" style="position:absolute;top:16px;right:12px;display:flex;flex-direction:column;align-items:center;gap:2px;z-index:2">
               <img :src="brandQrImage" style="width:36px;height:36px;object-fit:cover;border-radius:4px;box-shadow:0 1px 4px rgba(0,0,0,0.1)" />
@@ -189,9 +188,11 @@
                 <span style="color:rgba(255,255,255,0.85);font-size:8px;letter-spacing:0.5px">配方总成本</span>
                 <span style="color:white;font-size:12px;font-weight:700">¥12.50</span>
               </div>
-              <!-- Date: centered -->
-              <div style="margin-top:8px;text-align:center;font-size:7px;color:var(--text-light);letter-spacing:0.5px">
-                制作日期：{{ new Date().toLocaleDateString('zh-CN') }}
+              <!-- Logo left + Date right -->
+              <div style="display:flex;justify-content:space-between;align-items:flex-end;margin-top:8px">
+                <img v-if="brandLogo" :src="brandLogo" style="height:18px;object-fit:contain;opacity:0.5" />
+                <span v-else></span>
+                <span style="font-size:7px;color:var(--text-light);letter-spacing:0.5px">制作日期：{{ new Date().toLocaleDateString('zh-CN') }}</span>
               </div>
             </div>
           </div>