fix(ui): badge sizing, button alignment, country XX format, date-only fields

app/templates/postcard_form.html

@@ -19,7 +19,7 @@
         <label>收件人</label>
         <input type="text" name="recipient_name" value="{{ postcard.recipient_name if postcard else '' }}">
         <label>国家/地区</label>
-        <input type="text" name="country" value="{{ postcard.country if postcard else '' }}">
+        <input type="text" name="country" value="{{ postcard.country if postcard else '' }}" pattern="[A-Z]{2}" title="两个大写字母（如 JP、US）" maxlength="2" style="width:80px;text-transform:uppercase">
         <label>寄出时间</label>
         <input type="date" name="send_time" value="{{ postcard.send_time.strftime('%Y-%m-%d') if postcard and postcard.send_time else '' }}">
         <label>到达时间</label>