feat(home): 首页展示位空数据时渲染内置渐变图兜底

/showcase 返回空时 hero 拼贴与作品墙不再留白——
用 public/showcase 下 11 张内置 SVG 渐变图填充；
接口有数据时仍优先使用后台配置的展示图。

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

frontend/public/showcase/hero-2.svg

@@ -0,0 +1,13 @@
+<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="#0ea5e9"/><stop offset="60%" stop-color="#6366f1"/><stop offset="100%" stop-color="#1e1b4b"/></linearGradient>
+    <radialGradient id="b1" cx="0.3" cy="0.25" r="0.7"><stop offset="0%" stop-color="#bae6fd" stop-opacity="0.5"/><stop offset="100%" stop-color="#bae6fd" 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>