feat(hero): replace banner with full-frame architecture illustration

The previous hero-banner was composed as a wide banner with an empty left
40% reserved for overlay text. In the 50/50 hero column it rendered as a
shrunken half-width picture with dead space on both sides.

New illustration fills the whole 16:9 frame edge to edge (code/CI/CD left,
container + host cluster centre, observability right) so the right hero
column reads as a system diagram at its native 518px width, with the
2048w lightbox for detail reading.

Also add a 1280w WebP so DPR2 desktops do not pull the 2048w asset on
first paint.

src/pages/index.astro

@@ -73,7 +73,7 @@ const featuredMetrics: Record<string, string> = {
         aria-label="View full architecture diagram"
       >
         <picture>
-          <source srcset="/images/hero-banner-2x.webp 2048w, /images/hero-banner.webp 1024w" sizes="(max-width: 960px) 100vw, 560px" type="image/webp" />
+          <source srcset="/images/hero-banner-2x.webp 2048w, /images/hero-banner-1280.webp 1280w, /images/hero-banner.webp 1024w" sizes="(max-width: 960px) 100vw, 560px" type="image/webp" />
           <img
             class="hero-banner-img"
             src="/images/hero-banner.jpg"

src/pages/zh/index.astro

@@ -74,7 +74,7 @@ const featuredMetrics: Record<string, string> = {
         aria-label="点击放大架构图"
       >
         <picture>
-          <source srcset="/images/hero-banner-2x.webp 2048w, /images/hero-banner.webp 1024w" sizes="(max-width: 960px) 100vw, 560px" type="image/webp" />
+          <source srcset="/images/hero-banner-2x.webp 2048w, /images/hero-banner-1280.webp 1280w, /images/hero-banner.webp 1024w" sizes="(max-width: 960px) 100vw, 560px" type="image/webp" />
           <img
             class="hero-banner-img"
             src="/images/hero-banner.jpg"