feat(public-frontend): 公开前端 UI 视觉升级

引入统一品牌主题(色阶/字体/阴影/动效),重做首页 hero、卡片、导航与页脚,统一各页面的卡片/输入框/按钮样式。

- tailwind: 新增 brand 色阶、Inter 字体、card/glow 阴影与 fade-up 动效
- index.html: 引入 Inter 字体与 theme-color/description meta
- index.css: 双径向渐变背景 + @layer 组件类(.card/.input/.btn-primary 等)
- App/Home/ProductView/Contribute/ApiDocs/Account: 套用新设计系统

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

public-frontend/tailwind.config.js

@@ -1,6 +1,55 @@
 /** @type {import('tailwindcss').Config} */
 export default {
   content: ["./index.html", "./src/**/*.{ts,tsx}"],
-  theme: { extend: {} },
+  theme: {
+    extend: {
+      colors: {
+        brand: {
+          50: "#ecfdf5",
+          100: "#d1fae5",
+          200: "#a7f3d0",
+          300: "#6ee7b7",
+          400: "#34d399",
+          500: "#10b981",
+          600: "#059669",
+          700: "#047857",
+          800: "#065f46",
+          900: "#064e3b",
+        },
+      },
+      fontFamily: {
+        sans: [
+          "Inter",
+          "system-ui",
+          "-apple-system",
+          "Segoe UI",
+          "Roboto",
+          "Helvetica Neue",
+          "Arial",
+          "PingFang SC",
+          "Microsoft YaHei",
+          "sans-serif",
+        ],
+      },
+      boxShadow: {
+        card: "0 1px 2px rgba(16,24,40,0.04), 0 4px 16px -8px rgba(16,24,40,0.10)",
+        "card-hover": "0 10px 30px -10px rgba(16,24,40,0.18)",
+        glow: "0 12px 40px -12px rgba(5,150,105,0.45)",
+      },
+      borderRadius: {
+        "2xl": "1rem",
+        "3xl": "1.5rem",
+      },
+      keyframes: {
+        "fade-up": {
+          "0%": { opacity: "0", transform: "translateY(8px)" },
+          "100%": { opacity: "1", transform: "translateY(0)" },
+        },
+      },
+      animation: {
+        "fade-up": "fade-up 0.4s ease-out both",
+      },
+    },
+  },
   plugins: [],
 };