refactor(styles): enhance glass effects, card designs, and transitions across components

- Replaced `client:only="react"` with `client:load transition:persist="header"` for `GlassHeader` to improve page transitions and header consistency.
- Updated card and surface designs with refined colors, borders, and shadows for a cleaner, modern look.
- Adjusted CSS variables for better contrast and distinct layers.
- Applied consistent blur and backdrop styles across components for visual cohesion.

src/pages/index.astro

@@ -43,7 +43,7 @@ const keyFacts = [
 ---
 
 <Layout title={pageTitle}>
-  <GlassHeader client:only="react" lang={lang} />
+  <GlassHeader client:load transition:persist="header" lang={lang} />
 
   <main class="min-h-screen">
     <div class="relative overflow-hidden">