feat(home): 去紫色,改中性暖白底 + 墨黑主色

整页底色 #fafaf8，按钮/CTA/导航激活态改墨黑，
标题渐变改黑灰，占位图换低饱和纸感色板，
去掉彩色光晕与漂移光斑。

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

frontend/public/showcase/hero-4.svg

@@ -1,13 +1 @@
-<svg xmlns="http://www.w3.org/2000/svg" width="600" height="800" viewBox="0 0 600 800">
-  <defs>
-    <linearGradient id="g" x1="0" y1="0" x2="1" y2="1"><stop offset="0%" stop-color="#10b981"/><stop offset="60%" stop-color="#0ea5e9"/><stop offset="100%" stop-color="#0f172a"/></linearGradient>
-    <radialGradient id="b1" cx="0.3" cy="0.25" r="0.7"><stop offset="0%" stop-color="#a7f3d0" stop-opacity="0.5"/><stop offset="100%" stop-color="#a7f3d0" stop-opacity="0"/></radialGradient>
-    <radialGradient id="b2" cx="0.75" cy="0.75" r="0.6"><stop offset="0%" stop-color="#ffffff" stop-opacity="0.28"/><stop offset="100%" stop-color="#ffffff" stop-opacity="0"/></radialGradient>
-  </defs>
-  <rect width="600" height="800" fill="url(#g)"/>
-  <rect width="600" height="800" fill="url(#b1)"/>
-  <rect width="600" height="800" fill="url(#b2)"/>
-  <circle cx="432" cy="240" r="54" fill="rgba(255,255,255,0.5)"/>
-  <path d="M0 496 Q 180 400 330 496 T 600 440.00000000000006 V 800 H0 Z" fill="rgba(255,255,255,0.12)"/>
-  <path d="M0 576 Q 210 480 360 576 T 600 528 V 800 H0 Z" fill="rgba(255,255,255,0.1)"/>
-</svg>
+<svg xmlns="http://www.w3.org/2000/svg" width="600" height="800" viewBox="0 0 600 800"><defs><linearGradient id="g" x1="0" y1="0" x2="0" y2="1"><stop offset="0" stop-color="#e6c9bd"/><stop offset="1" stop-color="#c07a5f"/></linearGradient></defs><rect width="600" height="800" fill="url(#g)"/><circle cx="558" cy="256" r="60" fill="rgba(255,255,255,0.55)"/><path d="M0 528 Q150 400 300 512 T600 463.99999999999994 V800 H0Z" fill="rgba(0,0,0,0.12)"/><path d="M0 624 Q180 528 360 624 T600 576 V800 H0Z" fill="rgba(0,0,0,0.16)"/></svg>