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/work-5.svg

@@ -1,13 +1 @@
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-  <defs>
-    <linearGradient id="g" x1="0" y1="0" x2="1" y2="1"><stop offset="0%" stop-color="#6366f1"/><stop offset="100%" stop-color="#22d3ee"/></linearGradient>
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-    <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="800" height="534" fill="url(#g)"/>
-  <rect width="800" height="534" fill="url(#b1)"/>
-  <rect width="800" height="534" fill="url(#b2)"/>
-  <circle cx="576" cy="160.2" r="72" fill="rgba(255,255,255,0.5)"/>
-  <path d="M0 331.08 Q 240 267 440.00000000000006 331.08 T 800 293.70000000000005 V 534 H0 Z" fill="rgba(255,255,255,0.12)"/>
-  <path d="M0 384.47999999999996 Q 280 320.4 480 384.47999999999996 T 800 352.44 V 534 H0 Z" fill="rgba(255,255,255,0.1)"/>
-</svg>
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