improve: Silver スポンサーのロゴサイズと間隔を調整

- ロゴサイズを w-60 sm:w-80 に拡大（Gold より小さく In-Kind より大きく）
- 間隔を gap-8 に統一（全画面サイズで 32px）
- リンクありとリンクなしのサイズを統一

スポンサー階層: Diamond > Platinum > Gold > Silver > In-Kind
ほぼ等間隔（80px）でサイズが段階的に変化

Author: yasulab

_includes/sponsors.html

@@ -82,15 +82,15 @@ <h3> ゴールドスポンサー </h3>
     <section class="sponsors mb-12">
       <h3> シルバースポンサー </h3>
     </section>
-    <div class="flex flex-wrap justify-center items-center gap-8 sm:gap-4">
+    <div class="flex flex-wrap justify-center items-center gap-8">
       {% for sponsor in sponsors_silver %}
       {% if sponsor.link != "" %}
       <a href="{{ sponsor.link }}" target="_blank" rel="noopener" class="block transition-transform hover:scale-105">
-        <img src="/img/contests/sponsor-logos/{{ sponsor.img }}" alt="{{ sponsor.name }}" class="w-40 sm:w-56 h-auto max-h-32 object-contain">
+        <img src="/img/contests/sponsor-logos/{{ sponsor.img }}" alt="{{ sponsor.name }}" class="w-60 sm:w-80 h-auto max-h-32 object-contain">
       </a>
       {% else %}
       <div data-proofer-ignore class="block transition-transform hover:scale-105">
-        <img src="/img/contests/sponsor-logos/{{ sponsor.img }}" alt="{{ sponsor.name }}" class="w-28 sm:w-40 h-auto max-h-32 object-contain">
+        <img src="/img/contests/sponsor-logos/{{ sponsor.img }}" alt="{{ sponsor.name }}" class="w-60 sm:w-80 h-auto max-h-32 object-contain">
       </div>
       {% endif %}
       {% endfor %}