feat(branding): 启用新版 SVG logo 并在 README 头部展示

- 新增 assets/logo.svg 作为项目默认 logo（512x512 深色圆角底 + 渐变 S 标）
- 前端 favicon 与组件兜底 logo 由 logo.png 切换为 frontend/public/logo.svg
- 三个 README（EN/CN/JA）头部新增居中 logo 与标题布局
- 删除旧 logo.png 及多余的 lobe 变体 SVG

frontend/src/components/layout/AppSidebar.vue

@@ -14,7 +14,7 @@
         class="sidebar-logo flex h-9 w-9 items-center justify-center overflow-hidden rounded-xl shadow-glow transition-opacity hover:opacity-80"
         @click="handleMenuItemClick(homePath)"
       >
-        <img v-if="settingsLoaded" :src="siteLogo || '/logo.png'" alt="Logo" class="h-full w-full object-contain" />
+        <img v-if="settingsLoaded" :src="siteLogo || '/logo.svg'" alt="Logo" class="h-full w-full object-contain" />
       </router-link>
       <div class="sidebar-brand" :class="{ 'sidebar-brand-collapsed': sidebarCollapsed }" :aria-hidden="sidebarCollapsed ? 'true' : 'false'">
         <router-link

frontend/src/components/layout/AuthLayout.vue

@@ -33,7 +33,7 @@
           <div
             class="mb-4 inline-flex h-16 w-16 items-center justify-center overflow-hidden rounded-2xl shadow-lg shadow-primary-500/30"
           >
-            <img :src="siteLogo || '/logo.png'" alt="Logo" class="h-full w-full object-contain" />
+            <img :src="siteLogo || '/logo.svg'" alt="Logo" class="h-full w-full object-contain" />
           </div>
           <h1 class="text-gradient mb-2 text-3xl font-bold">
             {{ siteName }}