fix(ui): fix button text wrapping, upload layout, and image placeholders

app/templates/postcard_detail.html

@@ -17,6 +17,8 @@
             <h3>正面</h3>
             {% if postcard.image_front %}
             <img src="{{ postcard.image_front }}" alt="正面" class="detail-img">
+            {% else %}
+            <div class="detail-img placeholder">📷 未上传正面图片</div>
             {% endif %}
             <form method="post" action="/postcards/{{ postcard.id }}/image/front" enctype="multipart/form-data" class="inline upload-form">
                 <input type="file" name="file" accept="image/*" required>
@@ -27,6 +29,8 @@
             <h3>反面</h3>
             {% if postcard.image_back %}
             <img src="{{ postcard.image_back }}" alt="反面" class="detail-img">
+            {% else %}
+            <div class="detail-img placeholder">📷 未上传反面图片</div>
             {% endif %}
             <form method="post" action="/postcards/{{ postcard.id }}/image/back" enctype="multipart/form-data" class="inline upload-form">
                 <input type="file" name="file" accept="image/*" required>