修复 Hero 满屏高度的 calc 语法

Tailwind 任意值里 calc 的运算符两侧需用下划线代表空格，
否则生成非法 CSS 被丢弃。改为 calc(100svh_-_4rem)。

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Li-Evan

site/src/pages/index.astro

@@ -257,7 +257,7 @@ const dotGrid =
   </header>
 
   <!-- ── Hero (fills the first screen: nav + hero = 100svh) ── -->
-  <section id="top" class="relative bg-stone-950 text-stone-50 overflow-hidden flex items-center min-h-[calc(100svh-4rem)]">
+  <section id="top" class="relative bg-stone-950 text-stone-50 overflow-hidden flex items-center min-h-[calc(100svh_-_4rem)]">
     <div class="absolute inset-0" style={dotGrid}></div>
     <div class="absolute -top-40 -right-32 w-[36rem] h-[36rem] rounded-full bg-emerald-500/20 blur-[120px]"></div>
     <div class="absolute -bottom-48 -left-24 w-[30rem] h-[30rem] rounded-full bg-emerald-700/10 blur-[120px]"></div>