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-3.svg

@@ -1,13 +1 @@
-<svg xmlns="http://www.w3.org/2000/svg" width="600" height="600" viewBox="0 0 600 600">
-  <defs>
-    <linearGradient id="g" x1="0" y1="0" x2="1" y2="1"><stop offset="0%" stop-color="#f59e0b"/><stop offset="50%" stop-color="#ef4444"/><stop offset="100%" stop-color="#7c2d12"/></linearGradient>
-    <radialGradient id="b1" cx="0.3" cy="0.25" r="0.7"><stop offset="0%" stop-color="#fde68a" stop-opacity="0.5"/><stop offset="100%" stop-color="#fde68a" 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="600" fill="url(#g)"/>
-  <rect width="600" height="600" fill="url(#b1)"/>
-  <rect width="600" height="600" fill="url(#b2)"/>
-  <circle cx="432" cy="180" r="54" fill="rgba(255,255,255,0.5)"/>
-  <path d="M0 372 Q 180 300 330 372 T 600 330 V 600 H0 Z" fill="rgba(255,255,255,0.12)"/>
-  <path d="M0 432 Q 210 360 360 432 T 600 396 V 600 H0 Z" fill="rgba(255,255,255,0.1)"/>
-</svg>
+<svg xmlns="http://www.w3.org/2000/svg" width="600" height="600" viewBox="0 0 600 600"><defs><linearGradient id="g" x1="0" y1="0" x2="0" y2="1"><stop offset="0" stop-color="#cdd6e0"/><stop offset="1" stop-color="#7f93ad"/></linearGradient></defs><rect width="600" height="600" fill="url(#g)"/><circle cx="336" cy="192" r="60" fill="rgba(255,255,255,0.55)"/><path d="M0 396 Q150 300 300 384 T600 348 V600 H0Z" fill="rgba(0,0,0,0.12)"/><path d="M0 468 Q180 396 360 468 T600 432 V600 H0Z" fill="rgba(0,0,0,0.16)"/></svg>